There was this DIY design I came across decades ago that I've been trying to re-find for years. It had 4 12-15" cheap paper cone woofers mounted 2 sets of face to face with both sets axially loaded and wired so that all 4 cones move together. The whole thing was put in a big loaf of bread shaped box and then some kind of bandpass something and vents to the outside. It was reportedly very good for this low low base stuff, not so good for higher end responsive bass stuff. Anyone have any pointers for where to look?