I did this on my first car, a '71 Camaro, back in the early seventies. I had an in-dash factory AM radio and wanted an 8-Track as well. I simply made up two u-shaped brackets using the stiff (industrial--not residential) kind of galvanized pipe strap (with the holes). The industrial kind is stiff enough to require neidng in a vice or around an anvil with a hammer and therefore holds its shape (and the weight of a tape player) quite well, while absorbing some of the road impacts. Some foam padding prevents squeaks and rattles. It worked fine for the three years I owned the car; only had to drop the glovebox door open to change the tape or track. Newer units with remotes would save that trouble. I didn't keep the glovebox (cardboard) though as there really wasn't room left over.
