I second the recommendation to cut the brim out of sturdy plastic--my first thought. You might look around your house and see if you can find some flexible, yet sturdy, plastic that you think might work. No sense in buying something special unless you absolutely have to.
A rafting guide, huh? Now there's a profession you don't hear about every day! Good luck with your hat. I hope you have plans to make several hats for him since it might take a while for one to completely dry out (especially the brim part because of it's double thickness).