I tried it, and it worked! Thanks for the help.
The green glue knocked down the ringing of the cabinet nicely. But there are still small panels that I didn't treat which still ring a little. I think the best thing is to build it from scratch with panels made ahead with layers of thin board glued together with GG. It only adds about 1/16" layer so 3 layers of 1/4" MDF with 2 layers of GG would be very very dead, about 7/8" thick.
I used blue masking tape to mask off a 1" clean zone at the edges of the board, then trowelled on the GG with small sawtooth notches, then peeled off the masking, and layed it on the speaker carcass, then tacked it with 1-1/4" staples at point where there is crossbracing in the speaker where it doesn't vibrate anyway. There was no leakage.
The damping effect increased noticeably over 3-4 days as the goo dried. Even after 6 months it's still like soft gum so it never really dries. Adhesion inside the thin gap will prevent any oozing.
The advantage to damping versus bracing with dowels or internal panels is that they do not reduce energy, they just change the resonant frequency, usually upward into the midrange. Adding mass lowers the frequency but does not attenuate. Damping is the only way to quiet it. Green glue is good stuff!
Thanks to Carl for 3 tubes. I used 1 1/2 tubes on this.
