I just built my second cooler with a heating element. I just blew the first one when I tested out my new contol panel (turned it on with no water, blew in seconds). Anyway, I had problems with sealing my first cooler. I tried every combo including gaskets on either side, both sides, no sides, etc, but it still leaked. I ended up re-enforcing it with aquarium safe silicone. Had very little leakage after that.
So, my second attempt has been a success so far. Drilled a hole small enough that I had to screw the element into the plastic. I added a gasket to the exterior side and put some tape dope on the interior threads and tightened it with the nut. So far, no leaks and I didn't have to add any silicone to seal. Not sure why this one went better, but wanted to share my experiences.