I swiped an idea from, I believe, Donsbolt. He had used electrical junction boxes to mount some 4" speakers behind the back seat. I did a similar thing but made some brackets that mount them to the bottom of the dash--back underneath the dash. They dont get in the foot-space and they can be slightly turned to project the sound in a particular direction if desired. I bought some moderately high end 4" speakers (JL) for them and they sound great. Even provide a nice mid-bass.
The electrical junction boxes are 4X4" and the speaker mounts-in my application-in the lid. The way they are mounted makes it just complicated enough that it would take a little time or special tools to remove them.