At one time, I was planning on using 1/4 inch AC ply for our walls. Chickened out when I saw how bad even the A side was and how inexpensive nice Baltic (Ukrainian, apparently) birch was in those days. However, the A side of the A/C ply could be filled with epoxy and filler for a smooth appearance, sanded, glassed--possibly even on the inside, and then painted. I'll bet it could be rather nice.
I don't have a solution for replacing the 1/8 inch BB for the curved surfaces, however. Might be appropriate to consider square drops, or foamies.
Tom