I created this basic plug to fill the holes in my Kobalt folding work table, but they should fit most 20mm chamfered bench dog holes.  I print them using Amolen metal-impregnated PLA so I can pull them out with a magnet, though I'll admit it takes a powerful magnet to get a grip on them.  The included photo is of plugs I printed on my Ender 5 Plus using a 6mm nozzle.