What I would like to do is design and build an enclosure from damped marine plywood to a custom shape to fit in to one of the few locations I can place the woofer.

That's a great option. It's possible that there are companies out there who've already done this and have pre-made enclosures for those odd locations. In the Crutchfield catalog, there's a company that has a line of these enclosures for American cars (mostly pickups). Perhaps there's a European company specializing in Minis?

I don't have any enclosure-building URLs handy, but Crutchfield has a book on loudspeaker design that I think you can order.

Tony Fabris
Empeg #144