Side-insert T-nut lock-in-place

Printable inserts that go in from the side and rotate into place with a 4mm hex wrench to hold M3, M4, or M5 T-nuts in place inside a 20-series extrusion. They're opposite handedness of the T-nuts, so the T-nut cannot turn counterclockwise when unscrewing something from it and thereby push the inserts out of the way.

Unlike spring-loaded T-nuts, these do not require access to the ends of the extrusions. They can be used for adding things to an already-assembled machine.

Print in the orientation of the STL file to make the tensioning system work right. If you rotate it 90 degrees to print "upright", the tension will be sensitive to bed leveling/nozzle distance errors, and rotating the insert may shear off the tension bumps at the layer line rather than going under the lip smoothly. Preferably use a material not subject to "creep" (i.e. not PLA) if you want tension to be maintained long-term rather than just long enough for assembly. But if you just need them to hold short-term, PL