I have several designs that you could use for a subwoofer. Most are intended for a drivers like Omega's, Kilomax, Magnums or JBL Pro drivers. So they're high power solutions using drivers from about $150.00 up.

If you build your two π speakers in the tower configuration, I don't think you'll want for a subwoofer.

The push-pull plenum horn we've discussed here is probably a bit much for what you're wanting. But the push-pull idea could be easily implemented in something else. What I'd suggest you consider is a pair of Alpha 10's configured in a push-pull configuration in a box from 3.5ft3 to 11ft3 tuned to 40Hz. This could be a bass-reflex box or a π cornerhorn. If you have the corners, go for the latter. It's attractive and it performs very well. It will hide the exposed magnet so it's a nice way to implement the push-pull arrangement, in my opinion.

Wayne