I would cut out two pieces of .75" plywood on either side of the .75" built-up belting, and fasten that to another 3/4" plywood side board so that you wind up with a total thickness of 1.5" with a .75" wide by .75" deap dado. I am praying that 3/4" curved and glued built-up belting is rigid enough for a horn mouth, I mean, how rigid is the .375" bass horn curve in my A7?. I guess the top, back, and bottom of the speaker and dowels inside the loops will have to hold everything together? Maybe it will just have that Dean Martin sloppy drunk sound to it. I never said I thought this out that far.