Originally Posted by AnOldUR
The best way to get a good seal is to cut a clearance hole through the outter wall big enough for the body of the valve and a hole through the inner wall the size of the thread. Clear the insulation out so that the body of your valve will hit the inner wall. This way you are only compressing on a thin piece of plastic, not trying to squeeze two walls with insulation between them together.
Here's one with a plastic ball valve:
That's a good idea. I did a variation of this (or, took a different approach to fix the same potential problem): I dug out the insulation around the hole and filled it with Bondo. You can tighten all day and that part of my cooler isn't going to budge.
Seems like figuring out why it's slipping is key for you. Is the hole too big or do the cooler walls cave when you tighten? (Or something else?) Both problems can be fixed pretty easily, but require different solutions.