digital deigns would say 14 square inches per cubic feet so 63 square inches of port but you dont have to use that much if you dont have the room for it
I contacted SI about the BM Specs and there reply was:
"For the BM 12" drivers, aim for 2 cubic feet tuned to 34 Hz (per driver). Each driver displaces 0.1 cubic feet, and be sure to account for your port displacement (port surface area per driver should be near 20-24 in^2)."
So I'm Thinking a 4cd box tunned at 34Hz with dual center ports(ports 22in^2 a piece) would be ideal?