need help stopping a leak around my heating element

I have a converted an Igloo cooler into an electric HLT. Everything went fine, except that I can't get the seal around the heating element to stop leaking. I have tried it using the gasket that came with the element, I have tried some EPDM o-rings that I have at work just to test it and still leaks. it's just a small drip every 30 seconds or so, but I can't get it to stop no matter what I do. I am going to order a silicon o-ring today to see if that makes a difference. I really would prefer not to put silicon around the inside to seal it. I would love for someone who has had this problem tell me what they did to stop it.

What are you using to hold the element in place? The element has NPS not NPT threads, so that could be your issue there. I know http://bargainfittings.com/ sells a 1" NPS nut specifically for the heating element threads.

wyzazz, I have a machined copper nut. It screws all the way down. It seems like the thin plastic on the cooler may be distorting some maybe. I was thinking of doing a S.S. washer to maybe retain the shape of the plastic and try that.