The Las Vegas sun crumbled the 40 domed nut caps that came with our patio furniture set. My wife suggested as one of my first projects I design and print replacement nut caps. After printing one and verifying the fit I loaded up my build plate in Cura and let'er rip. I found that the cap fit varies from nut to nut. I could push a few of the caps on with my fingers but I needed to tap most on with a rubber mallet which gave a nice tight fit.
Although I have not tried it, with the right scaling this nut cap would likely work for SAE English sizes, too.
FYI: As designed, the inside measurement from face to face of this 6mm nut cap is 15.24mm. You can use this measurement to save you a step when scaling for other nut sizes. Divide your nut's facing flat sides measurement by 15.24 and multiply by 100 = Your scaling value percentage.
18mm / 15.24mm X 100 = 118.11%
After doing a test print tweak the scaling factor to further adjust for fit.
The hot pink nut cap (what the Chinese PLA manufacturers say is purple) is installed on my Anet A8.