incublinkus pretty much said it.

.75" MDF is the most popular. you want to caulk from the inside. if any of the sides are very long, more than about 2ft, i would use a brace to make it more sturdy.

when making calculations, remember to account for the space that the basket of the subwoofer take up. that spec is on the sub's spec sheet. and the same if you use braces.