The thing that scares me the most is the idea of the plywood coming apart under high RPM. And having mathematically demonstrated the speed at which the outer most edge of the plywood is traveling, getting hit with shrapnel from that could be quite dangerous. I got slammed in the gut by a 12" by 12" piece of 3/4 inch plywood that rode up and over the top of a spinning saw blade (RPM not readily known) and the black and blue area from the impact to just below my ribcage. It hurt like hell. And that piece of wood was only launched by the teeth of the blade and the energy it was able to impart to a small piece of wood catching the edges of the teeth.Producing round enough plywood disks just sounds quite challenging without a good lathe and a fair amount of skill.
Personally, I'd not use plywood. If anything I'd look into a 3D printed wheel with the appropriate size and RPM's for the speed at which I wanted to launch a PPB. Single wheel or dual. I think there's a lot more engineering that needs to go into this project than just imagining launching a PPB from a spinning wheel. The spinning material is the most critical component because it can be the most dangerous component. Much more dangerous than a shock hazard (unless the motor is 110VAC powered with a bad ground on wet grass). And other materials can also conduct electricity. I was shocked (no pun intended) to learn how conductive concrete can be at high voltages. Before learning that I would have stood upon my mothers grave swearing that concrete was not conductive.