Jeff,
Like you, I wanted the installation to be reversible...so I made two plywood boxes with open bottoms and sides contoured to fit the panel above the bulkhead behind the seats, and a slightly recessed front panel large enough to accept 6" x 9" speakers, and covered them in vinyl that matches the interior trim. I did drill two small holes on each side of the rear bulkhead to secure the boxes by means of screws inserted from the boot, but this disfigurement was about the least I could get away with.
The speakers sound pretty good, although the boxes aren't large enough to provide "bass reflex" volume. The positioning of the boxes makes getting to the boot release less convenient, but certainly not impossible; and obviously there's less room behind the seats to carry the odd item of luggage. The speakers fall just below the hood boot when it's fitted, so the sound isn't blocked off.