Chunky knobs made for a snug fit with M8 hexagonal bolts or nuts.  Modelled to work best with normal DIN standard fasteners (13mm across flats and head height 5-6mm to 6.4mm.)
Initially made for using with the partially-threaded bolts pictured; I decided to do some variations and this (fairly limited) set is the result.

Printing Notes

• Depending on how your nuts/bolts/printer/settings/material etc. the fit will vary - so try the test model first to find the fit you require, then alter the 'XY Size Compensation' by 0.1mm at a time to adjust if needed. I have only printed them in PETG so far but noticed a 0.6mm nozzle results in a tighter fit than with a 0.4mm. (different filament brands did not make a difference in this case.)
• Printing either way up is fine but use supports for the hex hole when that's facing the bed.
• 3 perimeters and 20% infill seems strong, but you can probably get away with less.