Thanks guys. There is nothing magical about it. Just use exterior grade plywood like everyone said. I needed 2pieces of 5/8"
to make up 1.25 thickness and just epoxied them together. Used a cardboard template to figure out what I wanted and then traced it onto the plywood and cut it out with a band saw. Clamped in the boat and drilled the necessary holes for the bolts. For rivets I bought closed end rivets from McMaster-Carr. I used Gluvit to seal everything up. My Knee brace was a piece of junk so I used it as a template and made up a new one out of 5/16" aluminum and then had a welder weld it up. Works great and wasn't as difficult as I thought it would be. Just take your time and make sure everything lines up.