Join Date: Dec 2001
Location: Belleville, Illinois
Thanked 22 Times in 17 Posts
The plastics that most of those things are made from are a type of polypropylene, closely related to milk jugs. It has a waxy feel and is darn near impossible for any adhesive to bond to securely. There are thermal glues that do a moderately good job, but they're not what you find at the hardware store. 3M makes the one I've used. I contacted an engineer via their website and got them to send me a sample. I've never found any other adhesive that works well.
If you wrap the whole underside in fiberglass and provide a means for it to mechanically attach, like drilling holes in the sides of the cups, you can get it to hold, but eventually water will get between the cup and the fiberglass and cause mold.
Probably the best repair would be to find replacement cups with a big rim, like the plastic cups that come in some can coolies. Then cut the cup out of the lid so the replacement is a snug fit. Put a bead of silicone caulk around the mating surfaces and press the replacements in. With a tight fit the caulk will adhere well enough to prevent leaks.