We went with roughly 3.6ft3 at 35hz with 52 sq inches of port.
Couple pics of a box I slapped together, still need to put the front boards on (double baffling it with 2 boards), carpet it, and waiting on the sub. Just a quick little box I threw together, has odd dimensions but this is how he wanted it lol... I would have preferred to make his box as wide and tall as possible, but he didn't want it very wide lol