Just a thought, but you might want to look into the ceiling mount enclosures that the Rodeo had in the rear. I recall the 2 ceiling enclosures in my '97 Rodeo that were mounted on the ceiling just behind the rear seats. Worked well with upgraded speakers, but anytime I was tinkering in the cargo space, I managed to bang my head on them. I think later newer year Rodeos actually had the speakers integrated into the rear ceiling panel so they were recessed.
I did a search for you but could turn nothing up. I think a more extensive search mught find soemthing though. I am quite sure there are aftermarket mounts as well that would be similar.
Just a thought.
Gary Noonan
'01 S/C VX / '18 Forester XT