Thank you for the replies everyone.
I took the fuse out and checked the clips for wiggle, they seem solid. I don't know if the fuse itself was cracked before or broke when I removed it, as it was REALLY in there on the right hand side of the panel, but the glass was cracked, so I replaced it with a new fuse.
Before I did that tho', I bent the right side clip down a smidge, then snapped the new fuse into place.
So far no issues with the heater going off... but will keep an eye on it.
Anyone know what size that fuse clip is, in case I need to know in the future?