Ask Bill.
http://www.billfitzmaurice.comparticularly, check into the Tuba 24. When loaded with the Eminence HL-10 (the smaller brother of the LAB driver), supposedly it is excellent through that range. You may want to put a subsonic filter on it at 35-40 Hz, though, to maximize headroom. For really light (musical instrument) duty, it can even work with a Beta-10, and demolishes a Peavey SP-series 18" sub in both loudness and extension, at the same time. I think I still have the AudioXPress magazine where details about the Tuba 24's design process are given. The plans aren't in there, though they're only $10 from the Bill F. website. Bill F. also designs W-style "Double Reverse" horns for musical instruments, but the Tuba 24 box can also find use as a sound reinforcement sub.
Its usefulness is limited by the fact that it is really only a single-10" even though its horn uses innovative design to fit enough horn into that space to provide the extension you need. The HL-10 is 8 ohms and has only 300 watts power handling, so it takes lots of amp to drive into those 8 ohms unless you stack two of them vertically per side for 4 ohms. At the price, though, you can afford to do that as long as you've got transportation. They should fit together well if you turn the upper one upside down so the two horn mouths integrate better.
Update: Bill Fitzmaurice says that his Tuba 24 horn, loaded with a HL-10 getting 300w, can be used to replace a dual-18" bin getting 1000w, in terms of output. So for those looking for a smaller alternative to something as big as the LAB sub, the Tuba 24 (because its external dimensions are 24"x24"x24") with the HL-10 should fit the bill nicely as a SR sub.
P.S. When you check out the BillFitzmaurize web site, and the Tuba 24, make sure you recognize that the response graphs shown are using a Eminence Beta-10, a very lightweight 10" speaker. Not the HL-10. According to Bill F, the HL-10 outperforms the Beta-10 by about 7dB at 40 hz with 200 watts in, in the Tuba 24. Also according to Bill F. it can really keep up with the major-league mains that are on the market, since it was designed to be paired with his very high efficiency DR250a. The biggest thing the Tuba 14 has going for it is its small size. 24"x24"x24". That's tiny.
I'm glad you guys are doing this. I don't think the HL-10 gets enough attention to really be as good as it can be.