Roof corrugated fiberglass-reinforced plastic (FRP) panels. They come in 8' x 26" sheets, are lightweight and relatively inexpensive - I paid $16 per sheet at a local building supply store. I opted for clear (i.e. not tinted) panels, as I wanted to maximize the light. One mistake I made was using 4-ounce, rather than stronger 8-ounce panels.