You might want to check out McMaster part 5530K23. It is a PTFE seal nut for the element.
I just did this with a cooler for HLT. It took a couple of times, and some help from The Pol.
The key (for me) was the following:
1. Drill a 2 1/2 hole in the bottom of the cooler BE CAREFUL to stop before reaching the inside! I let the pilot drill though into the cooler.
2. Using a 1" hole saw, FROM THE INSIDE, use the pilot hole and drill the hole. Go Slow to prevent splitting the bottom of the cooler (don't ask).
3. put the element in. Use the supplied gasket on the OUTSIDE and nothing on the inside.
4. Use a water heater element tool to tighten the nut.
5. Leak check 10 gal.
6. Hook up electric, encase the electric in epoxy or silicone caulk.
On my keggle, I think I am going to use a welded in coupling. This should make it easier.