You could fill in the space with partitions filled with expansion foam. But you'll find that the Kilomax 15 will work very well in the box you have, as long as it's tuned to 40Hz. The tuning frequency is very important, particularly when the box is a bit oversized.The Kilomax 15 works very well in boxes from 3.03 to 9.0ft3, tuned to 40Hz. When going oversized, tuning a little bit lower is best, like 35Hz-38Hz. It's tough to get things that close, particularly since parameters shift from environment, operation conditions, etc. But still, tune for 40Hz and if going a little large, tune a little low.
Another option is to use the Kilomax 18 instead, which will love this big box. The Kilomax 18 is best in 8.0ft3 to 25.0ft3 cabinets tuned to 25Hz.