COROPLAST it's the common name of the "plastic cardboard" y'all talking about. Looked for full sheets of it at Home Depot and was shocked at the $50+ Per piece price. Also consider the amount of paint applications as each layer will build up and dull crisp edges, cleaning is possible but the chemicals used my also break down the coroplast.
We used steel letter plates at Nascor to label the wall packages. They required cleaning no less than every week but if you are only making a dozen or 2 signs, should be no big deal.