Well, a thick bead of PL400 is a pretty decent gusset all by its lonesome. I lay it in with a caulking squeegee..............

Correct, you want stiffeners running along the longest axis, same as a horn.

I use 1x3 boards on edge or similar size scrap plywood.

There's no real advantage to vertically staggering them and if you get them too close to one end you create a 'hot spot'.

Well, you're creating offset 'X' frame braces with the lumber , but if you prefer to make all those extra cutouts that's fine too. Me, I was always in a time crunch and never had any effective dust control system, so tried to minimize cutting as much as practical.

If used, minimizing how much they protrude in the airstream along the walls is to *maximize* pipe action.

Unless the bottom is raised up off the floor, doubling up the bottom is a waste of time/materials and even then, bracing is sufficient since all the pipe's pressure is concentrated at the closed end (top) and why the panel bracing is along its axis.

Except for the full length vertical brace to tie the top, bottom and all four sides together there's no need for any more tie bracing, i.e. the object is to not have any bracing near any corners as these areas are already self braced by having glued joints as well as unbraced areas too small to resonate with any consequence.

There is a mathematically 'perfect' position along a pipe's length to place the woofer for smoothest response, though while a good point, I disagree with MJK's axial length 'L' x 0.349; instead using a different formula that includes the pipe's cross sectional area (CSA), but never having compared the two, I don't know if the differences seen in sims using his software are audible except possibly with the most minimal of internal damping.

Regardless, since your HF horn is on top, unless you sit really far away, like > 25 ft, then placing the woofer right up at the top of the cab is best overall and it also maximizes pipe action, ergo max vent damping.

Yeah, believe it or not, I get excited every time someone builds cabs like I learned to do so long ago. By age 22 when I did my first pipe horn designs, I was already pretty jaded WRT to high performance/SQ sound, but these took them to another level of BLH-like without all the extra bulk/woodworking, yet with the smoothness of low Qt sealed with none of 'boom'/'flabbiness' of the BR cabs of the day.

Of more interest to me though was that such alignments caused much greater improvements with 'el cheapo' drivers and I'm glad that now with MJK's software, the various TLs and their mass loaded (vented) variants are the de facto alignments for so-called 'full-range' drivers and their woofer + super-tweeter variants.

For this we have RCA's Harry Olson's circa 1949 MLTL design for his LC-1 'full-range' driver to thank, though he just called it a reflex, and the college student working at Altec that mentioned it to me when I toured the plant that year.

GM