I use 1/8” clear acrylic or plexiglass. It’s available from most glass repair shops or many building suppliers in sheets up to 4’ x 8’, or they mostly will cut what you need. It cuts easily on band or tablesaw, and is coated both sides to protect the finish.
I sand one side and epoxy it to the bottom form by taping it in place and then laying some laminates above it as well as the pressure strip (to provide some thickness), then add the air hose and air, put it in the oven and you’re done. Remove the protective coat before you bake it, or the adhesive can be problematic when baked. Very nice super smooth surface.