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For the upper and lower rails I sacrificed two of my garden U-posts. Drilling off the space part on the bottom and cutting off the tabs.
With the plastic being so easy to install in the screen door, I think a good option for covering a DIY greenhouse would be to find a bunch of old window screens and install plastic in them. Tons of them get thrown away when people replace their old windows. You could probably rig up something so you could pull various panels for ventilation.
