I got an old set of kick panels and covered in masking tape, layered resin and fiberglass cloth. Then peel the cast off, make some speaker rings, hold where you want with hot glue and dowel rods. Then wrap in fleece and staple it down. Proceed to fiberglass over the fleece. Sand, bondo, paint. A couple screws to remove, and replace with the original kick panels.
You dont need speakers in the back, focus the sound stage up front, like a concert.
Study up on automotive sound system design ...I think you will find the sub in the trunk will give you plenty of low end with good kick panel speakers and a couple of tweeters in the center dash sound can be contoured using sound processors to deliver more than adequate results.
