You don't have to douse the whole thing with expoxy. The dimm assembly looks like this: https://guide-images.cdn.ifixit.com/igi/dpYyM4oeOLPPTdpF.hug...

Putting epoxy around the top and bottom edges (where the retention clips are) and the right edge (where the contacts are) should make it extremely difficult to dislodge, but not impact the thermal performance of the chips (the black rectangles).