I've seen Cadets with their VHF radios mounted in a variety of bad locations. This is largely due to the very limited distance between the instrument panel and the fuel tank. It's only about 6" and almost all radios are deeper than that. The radio in my Cadet is hung under the panel on the starboard side. I've never liked this setup from the day that I installed it.
I was looking at a radio made by MGL Avionics that will mount in a 3 1/8" hole and has a depth of only 2.2" plus the connectors. It also includes a built-in intercom. It only weighs 1/2 pound. This radio is not TSO approved; however it appears that that really isn't an FAA requirement for all applications. Lot's of aircraft radios aren't TSO approved, including the King KX-170B. Here's a link to the MGL website:
http://www.mglavionics.com/html/radios.html What do you think?