The booklet i got with the kicker said the maximum could be a 6.0 cubic ft box. Annd on top of that, i had trouble even finding many boxes for my sub. I could find 3 ft, and like the 5's, but nothing in between.

 

maximum, yes, most efficient, no.. Build your own. I could make you some simple box plans if you want to take on the task. after that its nothing more than cutting, gluing, and screwing it together.