Well, you have a few options:
- You could use regular thermal compound and find a way to secure the case to the heatsink so there is absolutely no movement whatsoever. Movement between the two will drastically reduce the effectiveness of the compound.
- Use thermal pads. This is not as good as thermal compound or epoxy, but better than nothing. And it should provide a good enough bond to keep the heatsink in place, although I'd also ziptie it down just to be certain.
- Use thermal epoxy. This IS removeable. All you have to do is put the heatsink and ESC in the freezer for about 20 minutes and then gently pry the two metals apart. It will pop off - I've done it several times. Some say a heatgun will work too, but I haven't tried that.
A heatsink cannot be too large. At some point, it won't help much, but it won't hurt either.