Airspace Problems with Punch P2's
 

I recently purchased two of the Punch P2 8inch subwoofer's. My problem is, is that on Rockfords website it says optimal airspace for a ported box is .65 cubic feet for this subwoofer, however the box that I have purchased is .84 cubic feet per chamber. I just wanted to know if this is okay to use or if it is too much airspace and could damage the sub. If it is too much, then could you this issue by putting something inside the enclosure that is rigid and takes up space, if so what? Thanks!