PC panels are a break-proof alternative for glass. They are far stronger than glass, cut easily with power tools and are just as transparent as glass. These panels weigh less than two pounds per square foot. The only disadvantage of PC panels is they scratch more easily than glass.
Many do-it-yourself greenhouse plans call for polycarbonate panels. These are plans you can buy or print out from online sources. Some call for glass, but will accommodate polycarbonate panels in lieu of glass if the thicknesses match.
If you have engineering or construction experience, you can design your own PC panel greenhouse. Because it can be easily cut into many forms, PC paneling fits into shed-style, hoop, dome or lean-to greenhouses.