You do not need any glue of clips, just wiggle the heatsink on to the thermal paste to smooth out any air bubbles.
The dirt cheap heatsinks usually weigh sweet F.A.
Pushing down too hard on the heatsink runs the risk of Silver, conductive paste spilling over on to adjacent electrical components and frying something when power is applied.
Then I usually run the box for a while and then unplug power over night and the paste cures.