This is a simple one-piece painting handle for miniatures with 40mm bases. It is designed for round-lipped bases like those used by Privateer Press, but it should also work with GW-style bases (depending on what's on top of the base). It requires no assembly, and no elastic bands or springs or anything like that.
I gave it a fairly wide base for stability and a long thin handle because that's my preference. There is a 10-ish mm hole through the center that can be used to install a magnet or a weight to increase stability.
I printed it upright with no supports. That meant there was minimal cleanup needed (although a quick sanding of the jaws is still recommended to make sure there's no rough patches that might mar your bases).
I used 100% infill for strength, and also for added weight to help stabilise it. If you print with lower infill you might risk compromising the strength of the arms.
I printed in PLA with 0.2mm layers. If you use a different material, you may find the arms have too much or too little tension. If you're happy to edit the file, you can adjust the tension by lengthening or shortening the arms.
I made this one for 40mm round-lipped bases because my GW painting handles don't seem to work very well with them, but I'm considering making handles for other base sized. Feedback is appreciated.
(EDIT: removed some unneccessary material.)