Depending on the game/art/plastic, and any protection used,
the decision would be between plastic and metal.
When I use nickel or Zinc plated acorn nuts, I never over tighten, and have disc protection below, and
sometimes a washer on top.
Ebay has a nice bag of 50 for $6.95 including shipping.
With Wikibuy and Ebucks, $6.80
ebay.com link: Fifty 50 6 32 Nickel Plated Acorn Hex Cap Nuts BCP240