Jason, the handle you asked about is just a short piece of aluminum strip bent into a triangle shape and fastened to the flashing lid with a 1/8" aluminum pop rivet.

I have had the IMUSA mugs start to drip at the rivet too. I just put the inside rivet head on the horn of my anvil and bump the rivet tight again.

I suspect that it helps that I cut the handle off the mug and in particular cut it off quite close to the lower rivet. Being cut off keeps the handle from working against the rivet and loosening the joint. I left the upper handle part a little longer to use as a tab to grip while pouring.