Plastic sheeting can be used, but will deteriorate fairly fast. Fiberglass or other rigid plastics are commonly used with great success. It comes in flat form or corrugated form. Heat retaining power is better than glass or plastic film coverings. In warm climates, a fiberglass greenhouse requires less cooling.
Light transmission is good.
Insulated panels of rigid plastic are available. Fiberglass can be put on a straight surface or a curved one. Some rigid plastics are UV resistant. Foundations for a greenhouse can be solid concrete or a more lightweight foundation. Many use railroad ties to attach their greenhouse frame to.