I designed plugs in tinkercad to fill the bench dog holes (5/8" holes) in my Harbor Freight Bench. I made them with a nut insert so I could screw in CNC style clamps. Details are in my video:
I added an metric M6 Insert and Knob version for those outside the USA. I originally used an M6 Bolt and Nut in the first design but then couldn't find a long enough bolt (2 1/2") in metric so I changed the design to 1/4-20 since I had many of those.
I added a 3/4 inch hole insert and a 20mm insert to fit a Workmate portable bench. Details in this video.