M8 dome nut
thingiverse
Looking for M8 dome nuts for my Mendel build, I discovered http://www.thingiverse.com/thing:27769 which features a nut with extra clearance inside. Printed in black PLA at 0.2 layer height and 50% fill, they appeared great. The threading wasn't perfect but an M8 tap fixed the issue. Fastened with a 13mm spanner, these nuts can handle quite a bit of force, although I'm only using them for cosmetic purposes in my build. Just be cautious not to over-tighten and split the dome.
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