I would clean both surfaces with some Arctic Clean or ISO, apply some good thermal paste, and just a dab of thermal glue in two of the corners to hold it on (if there aren't any clips). Use something like a small book to keep some pressure on while it dries. This way you'll get the thermal transfer properties of the quality HSC, and the glue should be enough to hold the sink on, but weak enough to come off again in the future without much struggle.
