Romans invented concrete - a mixture of mortar and gravel with rubble (can vary the weight by varying the size of the rubble). It is cheap, strong, and flexible and allows architects to mold different shapes (arches, domes).
Cold frames for fall use. Cold frames are a simple way to extend the growing season. They are basically a box with a covering to let light and heat in during the fall and winter months. It functions as a miniature greenhouse to extend the growing season. The length and width of the box can be adapted to any glass source and garden size, so it might make sense to find your top first and then cut lumber to accommodate the size of the light.