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I used G-Flex epoxy (CAD$50 for ~200 mL) to first fill the groove, then cover it with a layer of 2 inch fiberglass tape wetted through. This first layer wrapped over the top about 3/4 in. This epoxy sets up VERY slowly, so I was there for a couple of hours using a spatula to keep putting the epoxy back where it belonged.
Surface preparation is crucial to this, as virtually nothing sticks to polyethylene, not even this stuff, unless you get this part right. The first step is pretty obvious - sand it. The second step maybe a little less so - clean it thoroughly with rubbing alcohol. The third part I would have died of old age before I guessed what it was - flame treating. You take a propane torch, and gently wash it over the surface to be bonded. No more than two seconds in any one place. You are NOT trying to melt it. You are trying to oxidize it to chemically alter it so that the epoxy has a fighting chance to bond to it.
I added a second layer of fiberglass tape, this time with the top edge flush with the top of the tank. Another couple of hours of herding epoxy back to where it should be before it finally gelled. And pulling the fiberglass tape back up again after it slipped down.