I think a light layer with RTV silicone followed with epoxy for the rest of the cavity would balance the need for speed (curing time) and retain some degree of toughness. If it were fast, I'd use 100% RTV silicone, but it would take too long. Silicone dielecric grease would be nice(thermal conductivity), but may leak out before it "dries out" in high temps. A nice "chewy" potting material would be perfect, like the potting material used in Ford TFI ignition modules, and such. Chewy=tough.