I design stuff with a simple and free on-line program called Tinkercad, you can export designs as an STL file that is 3-D printer ready. A simple bezel design would take a few minutes. I started off using Voice-to-CAD, and AI generated designs, until I figured out how much easier Tinkercad is. I just watched a few tutorial videos on Youtube and the printer is pretty much plug and play. Honestly, I'm still re-wiring my brain to think of 3-D printing first, it opens up project possibilities that couldn't be done before.
There are literally thousands of free ready to print designs for all sorts of things, including some great gadgets and parts you won’t find anywhere else. Check out websites like Thingaverse. Some designs, such as boxes, can be adjusted to any size, they have working hinges, latches, and sealing gaskets. A myriad of materials are available, including ABS, Nylon, and TPU, a rubber like material for flexible parts like gaskets. That “sealed” the deal for me.
For mounting a new stereo in your larger hole, you could use JB weld to glue a couple of overlapping wood cleats on the backside of the hole for the mounting screws. Then make a bezel like Bill’s to fit.
Geoff