For the depth, make the sealed portion 8" deep, and the vented portion 20" (or whatever you have left over after wood thickness) deep. You won't be able to fit a huge port in there though, so i suggest going external. That will give you about
3ft^3 sealed and 7ft^3 ported, depending on other displacements. It's probably barely enough; if you can expand any of those numbers by even an inch or two it will help.