I was looking at a shallow H frame, but when I centered the baffle the huge magnet was going to stick out the back and look like crap. So I started looking at a U frame but started to get concerned that the driver was at the front, cost a bit, and I was worried about it getting damaged by accident. So I recessed the driver baffle 2 inches to give it some protection, hide the magnet, and still have a shallow enclosure. The calculated response for all three options (U, H, and offset H) was so close it really was not a big performance trade-off.
Final external size was 17.25 inches square and 11.75 inches deep that can get down into the 20 to 30 Hz range. Pretty compact for a 15" woofer going that low compared to a boxed speaker. Two per side run in parallel to produce enough SPL to match a Lowther full range driver. Since the bass units are paired yet independent, I have enough flexibility to use only a single unit and mate it with a different OB or boxed system. Or I can stack them to reduce the footprint on the floor.
I like the bass output, I love the Lowther sound, so it is a good simple pairing in my opinion.
