1/4" Thumb nut thing.

thingiverse

Just another clever solution for your printer or router or whatever.\nAlthough, this one stands out from the rest. The nut is completely enclosed and simply cannot be pressed into either end. Perfect for situations where things get tight and you're worried about it coming loose, or when an ugly nut is visible to everyone.\nDesigned specifically for a 1/4-20 (or -24) nut, but an M6 may also fit snugly, albeit a little loosely.\nI had printed multiple units and my dad couldn't figure out how I got the nut in there. Granted, they were black, so the insert was a bit hard to see, but still.\nNo adhesive is required; simply press-fit it all together. Print with supports, but print one first to ensure a proper fit, depending on your printer, some tweaking or scaling may be necessary. I printed about five units before they came together without excessive force. However, you will need to use pliers or something similar to get it in there. It is intentionally tight by design (who wants it to fall out?)