What I would do:
Put at least 5 layers on the back, 1", will make it stronger and smaller. To play above 50Hz the back wall cant be so distant of the front baffle. Steel corners glued and bolted and fiberglass the corners to make them stronger.
Make a 4-5" baffle, make a round wood end movable and start with small port and cut inch by inch. Make a steel tube support for the baffle, bolt it on the exact middle of the box and bolt and glue to the baffle.
The port size you have various pics around the forums.