This was actually very easy to make, given that you have a router table, doing this free-hand is out of the question unless your a router god or something...I tried, and failed (that's when I went and borrowed the table from a friend).
Tools/materials I used to make this:
(note: sorry for blurry pics, automatic zoom sucks)
Well, I wish I would of made some notes to keep about the dimensions of all the cuts and such, but I didn't....sorry fellow G-body folks. :) But with these pics it should be self explanatory as to what to measure and where. This piece has a lot of routing to do, its a good way to get better with using a router table. lol The center cutout was done by drilling a hole inside where the cutout needed to be and then using the jig saw to work your way around.
The way this is made, on the G-body cars this will fit flush with the rest of the bezel. However, since the bezel has a small chamfer on the edges it doesn't give it the perfectly flush look, but pretty close.
At first I was going to recess the head unit into this so the head itself was a flush fit with the bezel, but doing so means having a 1/16" of a lip to work with. The tolerances have to be very good. So instead of wasting time that I didn't have, I just left it like this. Thankfully it still looks good (and MUCH better than the 'generic' mounting kits out there).