Not sure where you are geographically, but if you are near a Home Depot then rejoice. We have jiggered up many solutions from there, in the case of flooring we got something called Boise Cascade Standard Carb P2. It is a thin yet rigid particle board, in sheets of about 3ft x 6ft. It is coated on one side with a relatively tough white finish, slightly glossy, which is good for bouncing light, as well as being easy to clean, hard to get dirty. It has worked out great, we got a half dozen sheets and tape them with white tape at the seams.