I'm not familiar with the Alpine 9886; without knowing what it looks like or how big it is, or even what car it goes in, I'm pretty much shooting in the dark here, but I'll give it a shot:
You could start by looking for plastics suppliers in your area (yellow pages, Yahoo yellow pages, other online search, etc.). Go see them, tell them what you want to do, and ask for help choosing an inexpensive piece of plastic from their scrap bins or off-size material. Buy more than you need -- it'll probably be cheap, since it's scrap. Ask where you can get a little solvent and glue.
Cut to size and shape, make a way to hold it in, and off you go.
Note that drilling plastic is a challenge if you don't take your time. Pull the bit back often to clear chips, and go slowly so you don't build up heat. Practice on scrap first.
Have fun . . .