i made one out of pvc last year
all we did was take a 8' 2x6 and cut it so the wood was angled. then just screw in the pvc from the bottom. we used like 1" pipe so it was easy to bend and no heat was necessary.
The first thing u need to do is bend the wood. There are a couple different ways to do it. You can slowly bend it more over time. Also u can bend several thin pieces of wood which will ben eisier. Then attach them. THe best option if u can is to steam the wood for a couple hours. Then it bends really easy.
yeah i have a natural rainbow that i made with a tree and that works fine, and i also have an all wood down flat box, but i think a bank c would be different because if you ride to high you will be right on the edge unless you bevel the shit out of it which would make it really hard to stay on. it may work i dont think i would want to make it without using like pvc or piping on the sides though