5/16ths and m8 nut

These two nuts are actually really easy to print if your printer is well calibrated and they work great. I only had the opportunity to test the 5/16ths but I'm confident the m8 nut is good as well.

They are totally standard dimensions and should work in the place of metal nuts, up to a certain strength of course. Stronger than you would think though.

Recommended layer height: 0.1mm
Recommended material: PLA for dimensional accuracy