Thermal adhesive should do the job fine, it'll come with instructions but you generally have to hold it in place for a few minutes, then it'll set and hold the heatsink on firmly. I've used it on old graphics cards a few times, where obviously the heatsink hangs down from the card, and haven't had any fall off yet! There's also thermal tape but that doesn't transfer heat all that well, and nor does it stick as well as adhesive. It's often used on things where there's at least a small a clamping force involved and thermal transfer isn't important - like underneath RAM heatspreaders and sometimes VRAM, although they're more often thermal pads.
If there are other holes and bending the hooks will let you use them, that's a viable option too though.
