I need to fix some copper (which is being used to conduct heat away from power LEDs) to the inside of a cast aluminium light fitting, where the aluminium casting will form the heat sink. The copper and cast aluminium surfaces will be a poor surface contact match, so a fair sized dab of heat sink compound will be used. However, what is a good glue to use to bond them, preferably something with some body as it will be used where the contact gap is significant (as I'll have the heatsink compound where the contact gap is closer). I don't want to solder the aluminium as the outside surface has a decorative finish and I don't want to drill it as it forms a waterproof protection for the electronics inside.
So I was thinking about silicone rubber, or araldite, or something else? Silicone would have the advantage (if it works) that I could cut through it if I wanted to disassemble and reassemble for any reason.