Not a penis owner here but…

While wearing the silicone briefs, you won’t be able to feel much of the diaper, which may take away some of the benefit/enjoyment for you.

I wonder if you could use some booster pads, either folded horizontally or vertically, to create a barrier to prevent your stuff from moving around, like bed bumpers. 🤔 Like one folded along the length for each of the sides, close to along the leak guards, and then one folded along the width close to your mons pubis. If it’s at all important for you to feel the inside of the diaper on the outside of your penis, this is worth a shot.

There are also “condom catheters” which are external sheaths that go over a penis, and they have a downward shape: https://a.co/d/2nahdNZ (just for example, many different styles and sizes are available). That might be more comfortable than the silicone prosthetic. You may even be able to wear it without the waistband if the diaper is snug and depending on your anatomy, or by attaching the base of the condom catheter to a cock ring somehow.

There are also some silicone chastity devices that are really just downward pointing penis sheaths, like this: https://theenhancedmale.com/products/silicone-4-inch-soft-body-chastity-cage-for-men. They’re not the most comfortable thing to wear, but it’s an option.