Silicone rubber sealant. Sticks great, conducts heat as well as thermal paste does, withstands heat better than thermal epoxy.
One of the PC cooling web sites listed how well different materials conducted heat, and silicone rubber was as good as thermal paste, and both were much better than thermal tape (my video heatsink ran about 3 deg. F hotter after I switched from tape to silicone rubber).
And if you ever have to remove the heatsink, it's a lot easier to razor through rubber than epoxy.