Awhile ago I started to get into painting mini characters for DnD, and couldn't find a holder tool I liked, then started using an old grip from airsoft with some sticky putty attached to it and it worked pretty well. I wanted more but didn't want to spend money on them so I modeled a close representation to what I had already, and it turned out pretty good.

It uses two each of an M5x30mm screw and M5 Nut. Depending on how accurate your print is, the nut may be press fit.

This can mount to a proper rail system but real grips are made from more durable material and have had much more testing involved, use this at your own risk if using it on a real firearm. (Just go buy a real grip)

This would work very well for props, airsoft, Nerf, etc… however.