You might try Fitzmaurice's forum if you're looking for a folded basshorn right away. The π basshorns won't be done until this summer. Too many projects, too little time.

Be aware that basshorns are intended for high output use, and they aren't necessarily high fidelity sound quality. It's tough to keep response from being peaky on a basshorn. They're just too small unless they have some help, like room corners or use in multiples. Even then, you don't see them having the same efficiency below 40Hz as they do above, so they usually have more output in the midbass. The wavelength of 30Hz is 40 feet, so basshorns don't get anywhere near wavelength dimensions. That's why for hifi quality, I probably wouldn't go with a folded basshorn.

If you're planning to use an Alpha 10 for a home hifi subwoofer, you might consider using it in a π cornerhorn cabinet. The DI increase from corner placement is 9dB, so that's pretty significant. Put the Alpha 10 in a 5ft3 chamber tuned to 35Hz, it will make nice flat deep response, and you won't need EQ like you would with a folded basshorn.