Not much adheres to polyethylene ....
The BEST is to go to a 'gasket supply or gasket shop' (also listed as 'sheet rubber distributors') and have them either cut you a gasket or buy sheet material and cut yourself .... out of EPDM rubber (EthylenePropyleneDiamineMonomer) which is the most compatible for fuel oil, then EPR (Ethylene Propylene Rubber) ... both also compatible with 'bio-diesel' .... then the lesser choices become Neoprene or BUNA (ultimately swells in contact with diesel fuel and swells extremely with bio-diesel .... sooner or later you WILL be using bio-diesel mixtures). With more widespread usage of ethanolated and 'bio' fuels ultimately ONLY EPDM/EPR will be suitable. Sometimes available is a compound called 'Santoprene' an elastic foam that has equivalent chemical resistivity to EPDM.To make such gasket material 'work' youll need to add a flange to the tank ... by making a flange/sister-flange with studs and a handhole cover plate .... all bolted together with a 'gasket' between to make the 'seal'. For all of the above see if you can get such material somewhere near 45 or 50 durometer (a measurement of the softness/pliability) if possible, the 'usual' is 70 durometer which may be difficult compress to make it 'seal'. Nothing will 'stick' to Polyethylene (its too 'waxy') ... not Permatex, not 5200, not epoxy, not paint. not 'joint compound', not grape jelly, etc. ...... NOTHNG!!! To seal PE you have to use a soft gasket. ;-)