Hi, it's me again...two ways that should work. 1st take some course sand paper to ruff up the area to be sealed, Home Depot has a product call "AMAZING GOOP" it's a plumbing contact adhesive. Check out my topic "Rain Barrel" by Apollo. One more tip, take a propane touch...heat from about 8 - 10 inches from barrel opening for about 30 - 45 seconds. Now take something like a soda can to form it into a flat surface, practice on a junk barrel.

2nd, I'm 99.9% sure that the "Pipe Siphon by Apollo" will work using your end plug on your barrel. Made from 3/4" PVC parts, maybe two 45 elbows instead of one 90 to make a easy flow turn at the plug.