the inverter (thats what you have) shouldn't even get warm, let alone hot, unless you're severely overloading it somehow.
my inverter, which also arrived sealed in a plastic box (no ventilation what so ever) does not feel any warmer than the surroundings, even after running for a few hours. mine is using a TO92 transistor, your transistor looks a bit different, perhaps yours is higher power?
if you want to test things, to see if it will overheat, try potting the unit with paraffin wax ... if there's some sort of problem and the unit heats up, it will free itself of the wax fairly easily
I agree with zevon that shrinkage would be a concern, but I don't think in this particular instance you have to worry much about it, these things are pretty rugged and assembled with all through-hole construction. If it were a bunch of tiny SMD components, it would be a different story.
I suppose if shrinkage is a terrible concern, encapsulate the unit first with silicone caulk, and then pot it using a hard epoxy ... the caulk will act as a stress barrier protecting anything from being crushed by the epoxy.
I'm not sure about the jbweld, other than what I can buy locally, but I read an open circuit checking its resistance with a DMM - I suppose it could be a different story with high voltage / high frequency involved.