Also, there was a Professional Series one π speaker in the 1980's, and it was one of the most popular models. So look for a JBL 2115, and if you can find a pair, they make a really sweet little speaker at only 0.6ft3. Last I heard, the only reconing kits available are 16 ohm versions with white cones. But that will do just fine.

Another midwoofer that would probably make a great little speaker is the Seas W18EX001. There was a thread about making a PiAlign'ed speaker based on this driver, and the numbers look excellent for a tiny little box, just 0.3ft3.