Looks like an 8:1 setup...3/16" stainless is fine. The plate itself is stainless, so why bother tracking down/pay for the hard to find/high priced Monel (I've paid ~$7/pop for a 1/4" monel from Murray's) when the stainless plate will also be in contact with your mast?

Just put on a coat of Duralac or TefGel to reduce electrolysis and it'll last 10-15 years.