Boog
I'd suggest using .250" baltic birch plywood. It is more $$ than other material, and might be harder to find, but it has advantages. It is very strong, will hold screws well, and will resist warping from high humidity conditions. You could check a high quality cabinet shop in your area, they might have smaller pieces and scraps big enough for you to work with, but they'd still want a few dollars since the stuff is pricey.
I've made new speaker boxes for one of my teens from BB plywood that look almost exactly like the stock boxes, except about one inch deeper. They're about five years old now, and are still solid, despite being held together with glue only. This in a compartment that sometimes gets very humid, if you know what I mean.
Ed