Re: question about NuArc exposure unit
Typically called "Chipboard", not the best, as Sam mentioned, if you have a sign shop near you, see if you can scrounge up a piece of 3mm or so Sintra PVC sheet (they may have other brands). rigid enough for using under a plate, we use something similar on our plateburner at work.
I'm doing photopolymer process with Toyobo plates and also with ImagOn film. And I'm using a NuArc exposure unit for exposing my plates. The person who set up the unit in our studio but a piece of pressboard on the bed of the unit, so it lays under your plate and film. When I say pressboard, I mean a stiff "paper" (like a thin matboard) that they use in stiff mailing envelops or that you might find when you unwrap a new dress shirt--sort of cardboard without the corrugation.