Try the extracted center segment of a 2 litre soda bottle. The tall straight clear bottles represents a cylinder which can be cut and flattened into a nice sheet. Surprisingly resistant to weathering. When gently heated, such as with an electric heat gun, the plastic will shrink fit to an extent. The lens from a flashlight with the bulb/led removed? A small glass jar?
16 years off the grid and lovin' it!