West Marine has an epoxy compound that works with polyethylene. It is the only patch material that I've ever used that hangs on. I applied the epoxy to a crack after drilling a very small hole on each end, and then applied a flat patch [2" x 6"] over the crack with a heavy application of epoxy in between. The epoxy was red, the tank beige and it wasn't pretty, but it worked for two years till I sold the boat. To my knowledge, it is still not leaking or I'm sure the guy I sold it to would have told me by now. There aren't any other "glues" that I know of that work with polyethylene.
2013 Silverado 2500 HD Duramax/Allison Tuned
Aviator Wright Flyer #1919
"First In, Last Out". 11B4Y
DAV, MOPH, VFW, 5/7 Cav,1/8 Cav Assoc.
11th ABN Pathfinder Co. N7MIP