I knocked one up out of scraps when the whole manual machine thing was the latest fad. It was interesting to try but I wouldn’t spend money on one. It isn’t all that similar to an actual manual, just gives you an idea of where to balance. You really need to take the chain off too as otherwise you ratchet the cranks back as you wobble up and down (not an issue when you are rolling along rather than static) and using the rear brake allows you to lock the bike in any position rather than bringing the front down in a hurry.
I didn’t use any ply, just some lengths of timber from an old roof.