1. Buy free air subs and make a baffle out of plexi-glass and mount that behind your seat. But the plexi would be mucho $$$. Probably not the best option.
2. Buy a pre-made bandpass box with a plexi enclosure. I'm not really fond of bandpass boxes. They are boomy and they aren't to accurate. Some people like them. It's all personal opinion.
3. You can build a regular sealed or ported box. And put a plexi window in the back of it.