This is a hex nut to fit on a 12-3 leadscrew, such as found on this surplus car seat adjuster motor: www.amazon.com/12Vdc-Right-Angle-Drive-Electric/dp/B005IR1NBA

This nut screws on the acme thread leadscrew with a reasonably close fit (no backlash I can feel). It is not (initially) loose but the above 12V gearmotor is easily strong enough to slide it smoothly along. I actually wanted the nut to fit very tightly on the screw, so I could sandwich a pulley between two of these nuts tightened against it, to fix it solidly against the shaft and turn the pulley as the shaft rotates. It needs some locktite or other glue to make that transmit much torque. Of course my part is still just ABS plastic and nothing like a metal nut. However a single acme 12-3 metal nut is around $75 from McMaster, so there's that.

EDIT: after threading it on and off a few times, my first try got pretty loose on the shaft. I have now added another version "AcmeNut12-3-Tight" using 11.5 mm as the pitch diameter instead of 12.0 mm, and removing the un-needed face bevel to get a little extra thread length. This version needed a wrench to thread on the shaft initially. After moving it some, it can now be turned with fingers, but it is still fairly tight. You'll probably want to use heavy walls and infill for strength.