A knob for either a M8 Bolt or a Nut.
The nut or bolt is snug but easy to press into place
Added a tight fit version where you probably have to print it in at least 50% infill and press it in place in a vise. Or you can heat the bolt up with a soldering iron and press it in place with the tip of the soldering iron. The nut/bolt well is a bit tapered to make it easier to press into place.
If you need it to be really sturdy, print with 3.2mm Shell thickness (with a 0.4mm nozzle) then the infill can be 20%
Best print result if printed upside down e.g. top side against the plate