Amongst hardcore DIY speaker builders, "Baltic Birch" is considered the best plywood. It is void-free, 13-layer 3/4", often in 5' x 5' sheets, though sometimes 4' x 8'. A great enclosure can be made of an inner box of Baltic Birch (with cross-braces also made of BB), surrounded by an outer layer of MDF, with a thin layer of constrained layer damping material (such as E-A-R Isodamp) between them.
