I would think any 8" brand would work, as long as it supports a free-air installation. Another thought I had was to have a custom fiber glass enclosure that mounts up to the underside of the rear deck. That would cost some $$$ though.
You could make a glass enclosure fairly easily with some marine foam (the name escapes me now) and some "structural" fibreglass. If you didn't think the foam would be stiff enough you could glass in some metal reinforcements (like they do in hot tubs).