Help with replicator mini castle nut smart extruder

I keep getting error 81 lately and turns out its the castle nut on the carriage. the set screw is coming undone causing it to spin the shaft which of course stops the extruder from being able to push filament.

I retighten the set screw and off I go again.

how do I get it to STAY tight?

also what is the damned size of that set screw. .050 is a hair too small (but enough to tighten it for a little while) and 1/16" is too big.

what is between those? ie what is the proper size for that little set screw in the castle nut.

is it safe to use a little loctite on it? I was tempted to put a tiny drop of superglue on it but I fear getting it into the set screw hex socket and then being unable to use a wrench to tighten it again.