"Arctic Silver" epoxy is (or was) popular for PC CPU /
GPU heat sink attach.
What kind of package and how the heat flows, may
be a concern. Putting the heat sink on a plastic package
that has no thermal features (slug etc.) may still leave
you an inferior thermal path. Look at heat spreaders
as a means to "involve" more of the package surface
if the sink is smaller than package outline (facing area
from paddle and leadframe to sink, over plastic thickness,
is the deal).