I’m trying to keep costs down and can’t do two Kappalites.
Then build the right sub for the job. The T24's usable response is only good to about 45-50Hz, for HT, this leaves a lot of bass left out.
My suggestion is a pair of Table Tuba. The drivers can be had inexpensively. It is bigger than the T24, by 6" per side, but gives much better response, down to 30Hz. Don't expect ground-shaking bass from any of the subs, as outdoors is the most difficult place to get bass response, but the TT does do pretty good.
Whatever design you do build and use, make sure you use at least 2. More would be better.