roller nut difficulties

Hello everyone,

I want to make my first medieval crossbow. The difficulty i have is making the rolller nut.
What can i use for suitable steel does anyone know where i can find such a large piece of steel?

I also thought about making one of hard wood, how much pound can it hold?
Maybe i can make one from a lamination from pieces of hard wood and steel plates and pin and epoxy them together.

thanks in advance