I'm not sure about your HLT, but when I built my MLT, out of a cooler, I bought a high temp, two part epoxy, to fill in around the valve assembly that replaced the drain valve. You might want to look at something like that instead. I can't remember the heat level, but it was well over the boiling point. Something like 300*. I just got it at the local hardware store.
On Deck - Crabapple Wine, Delirium Tremens
Primary - Surly Furious clone, Kolsch
Secondary - Hofbrauhaus Lager
Bulk Aging - Brewtopia Barleywine, Pomegranate Wine, Red Table Wine, Wild Fox Grape Wine, Blueberry Wine