About once a month someone asks how to glue polyethylene; and while a few people suggest epoxy, the net result is you can't glue polyethylene.
Well, it turns out you can. A local craftstore went out of business and I couldn't resist buying bags of stuff I didn't need at 80% off. One thing I bought was Loctite SuperGlue "All Plastics". Naturally I thought it was bogus, but for almost free I will try anything.
Tonight a poly knob on my universal gym broke; as I was tossing it out I remembered the glue. It worked! Don't know how long it will last (I certainly don't expect much) but so far it is solid.
So if you are desperate, it is worth a try.