Thanks man! Huge fan of your designs/subwoofer philosophy. Not quite sure what the low corner is on it. Your standard th18 should do 35hz from what I've read, my goal with this was to keep the same horn style pathway but upsize it for a 21" woofer. Technically it should finish at 34'' deep but I decided to keep the extra 2'' of ply and make a final "bend" at the front as to hopefully, better couple with the air in front of the box.
30hz is the main goal, everything else below is a bonus & I think it delivers in spades, giving it's size, it better! lol, it's also overbuilt as a prototype, the first 2 panels, the baffle and front face are both 1", then solid wood wedges were used to create the last front angle, spray foamed the reflector cavities, all in all it probably added 10 + pounds but I think it was worth it.
I definitely cant wait to see it's true impedance with the 152 when I finally get to testing it this fall (working on the road till then) I'll definitely/randomly send you the details when I finally get around to it