That was just a guess, but the sides are thicker than the plastic on the screw cap. Doesn't matter how thick it is once you chamfer the inside it's thin enough to screw the flange together.
I put sealant on the outside of the tank where the threads came through, then ptfe'd the thread and screwed the outer on tight. The ptfe'd the thread on the element and screwed in to the flange. ( that sounds wrong)