On a wooden hinged frame, try corrugated plastic roofing. Sufficient lightness can still work as a door or hatch. Use standard roofing material (tin or asphalt shingles) and outdoor-rated plywood with a waterproof layer underneath, as others suggested. It only needs to provide structure for the actual roofing material from the Indianapolis roof company; you could probably get by with pressure-treated or cedar planks as well. Pressure-treated plywood and paint will suffice, at the very least, for a few years. It's common for people to sell or give away leftover project materials.