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.