The Rocket sub
One reason for this project (beside the other 50 projects I'm working on ;-) ) is Floyd Tool's book "Sound Reproduction" where he discusses multiple subwoofers in a room to diffuse or spread out room resonance modes to greatly improve low frequency response flatness and accuracy. There is also an excellent thread on this subject here, where, among others, Earl Geddes offers an empirical method to set up such a system with multiple subs.
The cone is about 80 cm high, with an internal volume of some 78 liters. I have designed a wooden adapter ring that accepts the excellent Peerless XXL 12 inch woofer. I had them painted in a body shop in BMW Arctic Gray...
A word about the enclosure: these are rejected missile nose cones from the HAWK air defense missile system. The material is glass fiber, about half an inch thick. They are very stiff constructions and yet relatively light (which is good if you have to loft it up into the air!). They have a resonance at about 1kHz; this frequency is well above the cut-off frequency of about 80Hz so it will never be excited. I've been using them for several years now and they produce strong, well controlled bass with an elegant form factor. Swords to ploughshares, isn't that the expression ;-) ?