I'm starting with a standard 15 gallon sanke keg
The element will go through the side via a 1" NPT coupling available anywhere. I got mine cheap from FleaBay
Now be aware that most elements are not NPT threads but NPS threads. The difference is in the taper. The element can be screwed into the NPT fitting a good way with a healthy dose of teflon tape wrapped around the threads. Brewers have been doing this for many years successfully.
More to come very soon