x2 yeah you should have plenty of room. i was thinking u said hdc318 at first and i was like hell no, then i say it said 15. i had mine in 4.5 which was a little too big. but worked pretty good
64^2 of port area for a 4 cube box and 18" long port for 38Hz. Around 1-2.5k of power would do well in a box that size. Built a bunch of them to that spec for daily...
4 cubes tuned 36-38 hz. For 36 hz, you could do 15^2 per foot but the best would be 16^2. So you're looking at 60-64 square inches of port probably 16-18" long.