Sorry for the delay in responding and the crappy pics.
When I first started this project, like you I was going to fabricate a frame to go completely around my Garmin GPS unit that fit flush in the cubby. I also wanted it to be easily removeable so that I could use the unit in my wife's car and have access to the SD card. I purchased styrofoam and black felt.
When I measured out the dimensions of the cubby and the GPS I discovered that the frame on top and bottom would only be about a 1/4" thick and the sides only about 1/2". Not really thick enough to support the unit or get the felt around.
What I ended up doing was fabricating a 1"x1"x3" (approximately) rectangle out of firm styrofoam then covered it with the felt. I then used it to wedge the unit in the cubby snug with the extra cable coiled up behind it and then leading out the corner which still allows the door to close easily.
This is an image of the rectangle removed. Goes in and out easily but still holds the unit firmly.
This is with the rectangle inserted. You can see the black power cord that wraps down and around to the 12vdc recepticle. The grey cord plugs into the accessories jack on the stereo. Allows the GPS voice commands to play through the stereo speakers when set to "AUX" along with bluetooth and music that I have stored on the inserted SD card.