This is a quick release mount for a Picatinny rail. It grips very tightly, but can be mounted and de-mounted quickly, without tools. There is a short version and a long one with double levers.

You can use this with my customizable mounting ring:
https://www.thingiverse.com/thing:3515747

And my customizable Picatinny rail:
https://www.thingiverse.com/thing:3481080

To make it, you need a M3x35mm cap-head screw, or two for the long version. (In fact,anything between 35 and 40mm long will work.) You will also need M3 screws to mount whatever you want to mount on top.

The mounting holes are 20mm apart, designed for M3 cap-head screws

Print this with at least 3 perimeters and plenty of infill.
(The lever looks like it needs supports in the gap, but I have found it works fine without. I suggest you try printing without, and re-print just that part with supports if you need to.)

Follow the video for assembly. I suggest you use a bit of glue to tack the screw in place. It's not essential, but the screw will work loose over time if you don't glue it.
https://youtu.be/0aoXJcprFGk

To mount it, engage the clip side first, and then roll the other side into place. Reverse to take off. (If you are having difficulty taking it off, you are probably twisting it. Keep the axis on the mount parallel to the rail, and roll it to the side.)

This can also be used without the levers, and with M3 nuts, if you don't want quick release.

For safety reasons, I don't recommend using this on a real firearm.