I recently modified a rolling tool cart into a multi-function workbench by machining a pattern of 20mm holes on the typical 96 mm spacing. I wanted to mount a small vice for light duty work. I therefore designed a set of tapered wedging plugs that can attach a baseplate to the hole pattern.

Because I expect to use and remove the plugs frequently I chose to embed standard steel nuts into the plugs. I used 5/16 inch nuts but M8 nuts are nearly the same size (slightly smaller) so they should also work well. A heat set threaded insert would also work but I didn't have any inserts in such a large size. To install the nut you simply pause the print at the appropriate layer (specifying a color change is one method in Prusa Slicer), insert the nut into the model, and resume printing.

The parts were modeled in the maker version of onShape and are available here.