The Neoprene Washer must be applied between the screw and the polycarbonate panel. It's made by galvanized steel with a layer of neoprene. The steel top of the gasket allow the pressure to spread around the washer, the neoprene, at the bottom, ensure sealing from water.
The gasket is 1" wide. Remember to not fasten the polycarbonate panels with the screws within 1/2" of the edge and to drill the polycarbonate with a diameter of 5/64" larger than the screw size. This will allow the a proper expansion of the panel.