M4 and M5  Knurled Knobs that can hold a nut or a bolt
These files include plugs for the Hex hole to help hold a nut or bolt in place.
One for each size has a hole in it for the bolt to pass through when using a nut.

I printed these with the Hex hole down and used support for the hex surface.
PLA 
infill 20%

The nuts and plugs were snug in the hex hole and did not need glue to hold them ( a light tap with a hammer helped).
I have included the DesignSpark file I used to create the Knurled Knobs incase someone wants to alter the files.