My original push nut broke when I removed the stitch plate cover from the lid.
I ordered 100 4mm push nuts from McMaster Carr for about $15. These fit the cover and I now have 99 to spare.
I found the easiest way to install these is with a 1/4-inch or 6mm nut driver. It fits over the outside of the push nut and makes it easy to push it down evenly.