Nylon or UHMW make excellent elastic stop nuts. If you want to use a plastic block inserted into a tapped metal block, machine an elongated slot into the metal block and use a rectangular piece of plastic. You need to have a positive means for preventing the plastic from rotating. You do not need to tap the plastic, but expect (some experimentation is needed) to drill the hole larger than normal tap drill diameter, and smaller than 6 mm. You may want to partially tap the plastic after installing in the metal block to make it easier to start the bolt.
Another technique is to put the plastic into the bolt. Cross drill the bolt near the end, and insert a piece of plastic. The plastic can be a tine from a comb, a piece of weed wacker cord, a piece of large diameter monofilament fishing line, or any small diameter ductile plastic. Poke the plastic through and cut off with a pair of diagonals (side cutters).
My father had an early model Ruger Super Blackhawk 44 Magnum. At the time, it was the most powerful handgun on the market, and it had a reputation for shaking the screws loose. He got tired of tightening the screws every time he shot the gun, so cross drilled all the screws and used tines from a comb as described above. It worked. The screws stayed tight. And he only used full loads.
Side note: I learned to shoot handgun with that gun. Same full loads, of course. I was in my early teens, and small for my age, so I used both hands to hold it. With a relaxed stance, every shot ended with the gun over my head, and me bouncing back about ten feet. Walk forward, load another round, shoot again. The other club members found the whole scene very entertaining.