I build speaker enclosures and have found roofing shingles to deaden sound just about as well as Dynamat, and it's cheap enough to double-layer. I use Liquid Nails in speaker boxes, but that would probably be a bit messy in a puter case
And besides, Dynamat works better for low freq (vibrations=bass), not fans (which are high-pitched). A better alternative for fan noise would be something like that puke-yellow foam egg-crate stuff you can get rolls of for putting under bedsheets (what the heck is it called?!). That stuff doesn't have any smooth surfaces (unlike Dynamat), so it will not just reflect "unwanted high freqs", it will absorb them. That would be a bit trickier to put in a case tho cuz it's about an inch thick. Maybe the thin stuff would work OK??? I would love to run some dB tests on some of that stuff, but my SPL meter doesn't read below 70dB