One inch thick. Three quarter is probably strong enough but the guy I bought the starboard from had a one inch piece handy. In plywood half inch would do.

Suggestion: If you make one, make the top edge with a piece that sticks up to go on the inside if the clamp. That way when you put the motor on the bracket it can't slide off. I use that to put my Tohatsu 3-1/2 two cycle on the stern rail from the dinghy. Then I don't have to worry about it slipping off before I get on deck to tighten the clamps.

My first thought when I read the original post was: what direction was the grain running. I saw that it was answered. Was that a commercial product?