You can use a spade bit a touch wider than any galvanised washer you might use,they are just a little too short
Shou Sugi Ban your bench and add some character!
Update: I've been experimenting with some granite tops that were given to me by a friend and even though they are slick, large bonsai pots stay in place no matter how much wind is thrown at them.Granite tops might be too slippery for the feet of the bonsai pots to grip properly, which may cause your trees to be blown off of the benches…
Then I have to see what the sun/shade spots are at various times of day.