Very interesting project, and beautiful workmanship. I never did see a link to the original plan? Or maybe there isn't one.

Two thoughts, both incompletely informed perhaps. The result of idle midnight thinking.

Sealing openable joints: I've thought of using ordinary Crouse Hinds (or other manufacturer.) duct seal (sometimes called Elephant Poop amongst other things) My thinking has been to run it thorough my old hand crank pasta machine with the linguini blade installed after thickness rolling. The stuff is good forever in some pretty tough industrial settings but provision would need to be made with a "pry" lip attached to the removable piece. Or maybe a screw thread against a hard spot on the support cleats to break the seal. It's pretty sticky. Drivers can be attached to the baffle using these: http://www.mcmaster.com/#threaded-inserts/=bzcrce Drill the mounting holes so they don't go through the baffle, or (if they do) back them with a piece of 1/8th ply, glued in place with some of GM's magic poly glue. Actual leaks through the driver suspension (if bad enough) might require re-doping the surround. Weber speaker might sell some speaker dope; I dunno.

Second. I've heard some good things about R-13/R/18 for internal sonic resistance and have seen it settle over time to the bottom of the box. My thinking has been to loosely tack bird netting (the black UV resistant kind) over the bats to try and hold them in place. I've heard that packing the stuffing too tight will require a larger box for the same results as loose filling but it may be worth a try.