I had some made from plexiglass by a local plastics shop. They were only for 11x14 paper though. The 1/4" plexy was too thin for this (slight bulge on the sides) so for 16x20 you really would need to go with something thicker. The edges and base could be the thinner stuff though. Two Nova clips worked fine for 11x14 fiber, I assume it would be OK for 16x20 as well.