Screw hooks into the fence, then use 1/2 or 3/4” PVC, with elbows on the ends & Tees every so far, then attach long pieces into those sockets, to construct a lean-too frame, and cover with heavy gage clear plastic. Maybe use thin strips of wood (tongue depressors or popsicle sticks) with staples to attach the plastic to the fence itself, both across the top, and down the ends. Anything to weight down the bottom edges, and you’re in business.
Okay, I “think” I understand this. What would I do about the sides of the lean-to?