Not polyuthene what a nightmare that stuff is, f44/46 and make sure you weld it properly and there's no problems, never had one fail in over 10 years
I do my share tbh always 44, not 46 as I don't have the time normally lol very rare to need epoxy (thankfully!) as above, get your welds sorted and your sorted
Mapei eco fix for me, put the sheet in cut round the door frame dry take it all out and then glue the lot out In one eco fix round the drain aswell