Hey Kats -
Yeah, hot-melt is definitely ONLY for 'positional-tacking' / temp-hold use -
especially for mission-critical alignment-components like a PBS cube - same for silicone - best
only for non-alignment-critical stuff like an AR-window, or to seal for dust / fog-proofing...
I find hot melt is also good for 'locking-in' wires to DB-XX shells, so, even with time / a strain-relief installed, despite lots of on/off's, twisting etc, they stay nice and 'potted'...(but not impossible to remove if need-be)... also good for electrically-isolating super-close proximity soldered-stuff / protecting electronically-sensitive areas from finger-touches; 'raw' / exposed 110V-points that you might encounter with a 'knuckle',

etc, while adjusting things while-running, etc, etc...
Best use a good, high-temp-resistant 5-min 2-part epoxy - Loctite / JB Quick, etc, etc... as has already been posted by several - I am simply adding a

to epoxy, and

for hot-melt / sillycone for alignment-criticals....
And while we're on the 'epoxy-channel' for a minute, the best applicators are those long wood-stick cotton-swabs - TIP:
save the used / dirtied ones when you finish cleaning your galvo mirrors

and when you need to apply epoxy to a real 'fine' / tiny area, 'snap' the dirty cotton-head off, and then 'snap' the stick at a
shallow angle, with a slight twist - you should get 2 nice, long 'slivers' that are perfect for applying epoxy to real-tiny areas
(like a galvo-mirror, or tiny PBS / lens, etc) - yeah, sure, you can use a 'toothpick' but this method gives you a nice, long 'reach' and a super-fine / needle-like applicator... just a friendly tip...
j