I used two of the RetroSound 3.5 inch speakers. Bought an aluminum plate with the two holes for the speakers, but you could use medium density of fiberboard (MDF) and cut out your own. See attached link where I show several pics of the install in my 71 Roadrunner.
Ron



http://www.roadrunnernest.com/forum/viewtopic.php?f=2&t=4431&hilit=RetroSound