A rule of thumb when building enclosures is that an average airspace range for a 12" sub is somewhere between 1.5-2 cubic feet (sealed) and 2-2.5 (ported). However, this is only a general rule and your sub might take something a little different. If you can't ask a dealer, I would stick with these specs. Sorry I can't give you the exact specs (I refuse to work with DHD, no offense).