Sometimes to do a clean install it would be nice to get big bass in a smaller space! I have never heard of any of this being done so I wanted to pose a few strange ideas and see what people think, or know about it!
1) Remote volume
Build a box just large enough to house the speaker, then use a fairly rigid tube to connect the air space to a remote resivior that has the volume needed to allow the sub to have the correct compressability to make the sub work as designed. I imaging that the surface area of the extension tube would be pretty important here.
2) Mimicking the air volume with a smaller flexible enclosure
Build a box just large enough to house the speaker, then mount a diaphragm of some sort that could be tuned via spring tension to flex/compress like the appropriate air volume would do. On the outside of this moving part, it would be shaped to be an extremely inefficient radiator so that it doesn't produce sound waves that interfere with the speaker.
Probably both totally nuts with a lot more problems than I considered. But hey, you guys here probably have tons more experience with the theory of operation for subwoofer enclosures than I do!