The red beret is closest to a Scots bonnet - except for the color! Throw it in a pot of coffee or tea and turn it brown? That would work, but it wouldn't dry in time.

A slouch hat with a belted plaid? A bit jarring, but you're at a N. American rendezvous, where it's doubtful anyone would be wearing a "great kilt" during that period anyways, so what the heck.

My suggestion would be to go hatless if it's not raining. You can probably pick up a decent bonnet from one of the vendors at the event....