I've experimented with the JB Weld as you described. I actually used a toothpick as a "plunger" to evenly distribute / pack the mixture. For that fact, I left the 'pick in the goop and trimmed flush. It certainly provided the necessary wall-to-wall strength to prevent a blow-out. My favorite stuff is the good marine epoxy....it can be tapped, sanded, painted, etc.I have filled a small hole from a tuner with wood jb weld epoxy and redrilled after it dried. Not the best way, but it worked.
Riis