how many inches is your sub? there is a ratio of sub diameter to air ratio for the box to use
edit: you can always get polyfill box stuffing and put some in your sub box. the point of the stuffing is that the stuffing changes the air volume density in a box, so the driver performs as if the box were bigger. use 1-1.5 lbs. of stuffing per cubic foot of volume.
*i seriously think cotton or polyester stuffing will do the same thing here
and be cheaper*
8" - .35 internal volume (cubic ft)
10" - .65 - 1.25 internal volume
12" - .85 - 1.75 internal volume
15" - 1.5 - 2.25 internal volume