Just build a better box. Find out how many cu ft. of air space you want on that sub. Then find the dimensions like AxAxA, make sure their in inches... then divide those by 1728. That'll give you the cu.ft of air space. When you find a set of dimensions that'll work then, there ya go. : ) Custom boxes are simple. Hence example, a box 12" long, 20" wide, 15" tall = 2.08 cu.ft. airspace. So with that figure out the dimension you want.... make it less lengthwise if you want leg room.