Try teflon tape on the screw, as many turns as makes it hold.
For my corexy I built a kinematic mount with the screws through a teflon block following Mark Rehorst’s advice. On my bed flinger I also made a kinematic mount, but just wrapped teflon tape around the screws. They’ve never budged. (I originally tried some loctite in the hole as well, which was not compatible with the teflon tape. I cleaned that out and used just teflon tape.) There’s a picture (upside down, during construction, sorry) here: