hiya
Im a plastics engineer by trade and yes you are correct almost no glue will
adhere to HDPE or PP (polypropolene) do you know which one it is ?

When I was an Old Town dealer they had a repair kit for their royalex canoes
that adhered very well. Know of some that were patched 15 yrs ago & still
holding fine. They had color matched goo in there that stayed put after you
prepped the area well with abrasive & solvent.
Greg Sefton