^^^ nice post!
Vacuum bag is a good idea; I haven't used it though.
I did try the "stacked" method of making a form, and had problems with surface cracking/impressions. I tend to think fiberglass over a clay plug is the best solution because the materials are relatively cheap and easy to source, easy to work with and they don't take any special tools or skills.
Ultimately though I think the "why" overrides any of it - nobody wants a plywood bass, and even if a laminated back was desired for whatever reason it might as well be flat!
