My partner Angie found a free geodesic dome play structure online — a tree branch had fallen on it and damaged several of the structural supports. I fabricated new ones out of EMT — they are the sections painted black in the photos below. i also built a door frame out of scrap lumber. In the winter, this can easily be converted into a greenhouse by taking out a section of the metal supports, installing the door, and covering it back up with greenhouse plastic. I was able to start loads of seeds in the greenhouse this past season, which resulted in much earlier yields and stronger seedlings that need less hardening off.