I find the idea of a more permanent-ish approach to stimulation very intriguing. And I love sounding. The concept of affixing a urethral sound to constantly poke the prostate is something I've been theorizing about for a long time. Lately I got to experiment a bit more with silicone sounds and those are very comfortable.
I commissioned these concept drawings because explaining these ideas with words is difficult. And because I like having concrete representations of interesting concepts.

Concept 1:
For "the mere human" I experimented with a bent piece of enameled copper wire around the glans that holds the silicone sound in place with a thin piece of latex tubing.
That does work somewhat well.
A neat side-effect is that the tip of the sound moves in and out with movement from outside. While erect, the tip sits against the internal urethral sphincter. Small movements, like crossing legs, clenching, or even sneezing, feel quite pleasurable. Bending the penis down pushes the tip through the sphincter, which feels intense.
But there are some downsides:
The copper wire interferes with my frenulum piercing (shown only in the 3D render) and tends to pinch skin against it. It took me a few tries to get it right, but now I have the wire wrap around the piercing. (There's an upside to this: The piercing holds the wire in place, so it's much less likely to slip. It also adds to the permanent-ness, because I have to take the piercing out in order to be able to take off the wire.)
When the penis is flaccid and hangs down, the tip of the sound reaches quite far into the bladder. When it's mostly empty, the tip pokes the far-side wall, which feels quite uncomfortable.
The bulkiness of the copper wire and outer part of the sound makes it difficult to find a good position in the underwear where it sits comfortably. The additional weight pulls everything down and makes the penis slip out of the underwear.
And it kinda gets in the way of everyday (and not-so-everyday) activities. Urine can flow just fine, but when it reaches the latex tubing, it tends to spray sideways. That, and the need to wash everything afterwards, makes using public toilets challenging, and it's annoying in general.
Something much less obtrusive is required.

Concept 2:
You might have heard of a "dolphin" or "deep PA" piercing. Usually, a deep PA would be placed in front of the scrotum, not at the perineum. But that has implications for how maneuverable the foreskin is. That section of the urethra is also more sensitive (not in the good way) in my experience.
So the plan is to get a deep PA at the perineum, let that heal off, and then use the holes to affix the silicone sound. That's about 11mm of tissue (when uncompressed) to pierce, which is quite a lot.
I devised a cylindrical adapter piece that's kept in place by two pins, screwed in radially. The one axial pin holds the silicone sound.
This solves basically all of the downsides of concept 1. It also makes the tip of the sound much more fixed in place, which unfortunately takes away some stimulation, but fixes the poking issue. And with some careful adjustments, I should get the tip into just the right spot, such that contractions move it enough to feel pleasurable.

Concept 3:
This, along with concept 4, is the adaption to the canine anatomy. There is a nice spot behind the knot where the piercing could be placed.
The tip of the sound then also moves with the erection. The prostate and bladder seem to be somewhat elongated in canines, compared to humans, and the knot itself does not move as far out of the sheath, so the poking issue would probably be less prevalent.

Concept 4:
Like concept 3, but mid-orgasm. And with a condom, because condoms are always fun.
When erect, the tip of the silicone sound is pulled into the prostate. And with everything squeezed in there, some leakage into the bladder is to be expected.

Concept 5:
This isn't directly related to the piercing / silicone sound, but I wanted to include it here because it fits thematically.
I've been playing with foley catheters for many years now. Also for their intended purpose (more or less), but I originally started because I wanted to find a more direct way to stimulate the prostate. Before that, I tried miscellaneous smaller toys, but reaching the prostate in a comfortable way was difficult with those. So in a sense, this is where my endeavor in prostate play really started.
It seems that I got lucky with the first batch of catheters I bought, because those were quite well-made. Later ones always have quite sharp edges at the opening where urine flows through, which makes inserting them somewhat painful. I always have to chamfer those edges with a nail clipper and then thoroughly clean and disinfect them. I eventually found silicone sounds, which are very comfortable, but they are not a replacement for the kind of stimulation a foley catheter can provide.
The balloon is supposed to be inflated in the bladder, to prevent the catheter from slipping out. But I was really curious to find out how it feels to inflate the balloon in the prostate. One of the best decisions of my life. It's no use trying to explain that feeling. Other than "intense" and "it feels very good" I don't really have words for it.
A slightly filled bladder causes a slight but definitely noticeable feeling of needing to pee. With the balloon big enough, that's not really possible, but that doesn't stop the body from trying when "letting loose". I don't know for sure what exactly is going on, but it feels like the urine is squeezing into the prostate next to the balloon. And that causes what I can only describe as the most intense stimulation I have ever felt.
Unfortunately, it's not really something I can recommend to others because it's such a hefty procedure. I've carefully tested my limits over the years, and not everything went smoothly (I'll spare you the details). But your body might react completely differently. I'm not a doctor. Please be careful with your body.

So... Why?:
There's the obvious part: Getting pleasure from stimulating the prostate feels good.
But aside from that, I've always found it interesting to have different ways of getting stimulation, other than just stroking the penis. Anything hands-free I find particularly appealing. The thought of doing "more", of finding something long-term, started out as a simple fantasy many years ago. From there, it kept evolving.
I'm not much into chastity, but I can relate to an aspect that I see in artworks that explore long-term/permanent chastity: The idea of having something unusual and exciting to occupy your thoughts during the bland parts of everyday life. Sure, it can be irritating at times, and then you'd rather take it off. But when it's not easy to do that, you'll just have to make do, and by sticking with it, you'll get rewarded later when it feels nice.
I have commissioned female art of myself in the past: https://www.furaffinity.net/view/49317995/ "I am male, and I feel content with that" That hasn't changed. But I can't help but wonder what it would be like. Stani asked me recently: If I could choose freely, and it would happen magically, would I additionally want to have female genitals? I would, because I imagine it would be a lot of fun. In reality, I wouldn't want to go through surgery, because it's not the same as "the real thing", but there is a parallel that, while it wasn't a driving factor before, now that I'm thinking about it, I find very appealing: One way to masturbate is to rub the clitoris. The piercing in concept 2 is positioned roughly in that area. I already like being touched/held there, but rubbing the outer pins of the piercing should transfer some movement into the prostate, which would make this especially pleasurable.
As I gained experience and found new tools, more and more of that fantasy became plausible. I'm currently at concept 1 and it's going great! Dealing with its problems is irritating at times, but it has given me a great amount of pleasure, and also comfort. For example, when work is frustrating, I can calm myself down by clenching my muscles down there and giving myself some good feelings. I've been looking into how to make concept 2 a reality, but there are a lot of problems to solve first. I'm having a hard time finding a piercing manufacturer who accepts customized requests like that. And I have to find a qualified piercing studio. Then there are some health considerations. So I've got a long way to go.

While there is nothing in these concepts that's specific to me, canonically speaking, I consider the canine depictions to be me, so I'll tag myself as the character.
Thank you, Kanrei, for putting my ideas to paper and for being so patient with me!
Also a little shoutout to siliconenozzles.com, where I bought the silicone sound (SSND-SBT 6-7.5mm 20Fr). Their website is a bit weird, but so far, all of my orders arrived.[\section]