I'm in the middle of a Spitfire stereo install as well. I think we even have the same speaker enclosures. However, I am mounting a power amp in the trunk to push them with hopes of actually being able to hear music at highway speeds. I'm mounting it to a new passenger-side trim panel made from plywood. I'm working on the template now.
Unfortunately, progress has been slow lately as I've been eyeball deep in working on MINIs...