Fabulous idea, Natalie. For ventilation, I'd suggest you cut a 'window' or two in the end opposite the door. Then if you can face the greenhouse with the door towards the wind, you just need to open the windows in the opposite end to get great through-ventilation. Save the need for a fan, and let the wind do the work.
Just cut 3 sides of a square opening - bottom and two sides - reinforce the edges with duct tape as suggested above, tape or glue a wood batten across the bottom and then when you need to ventilate, you just roll it up. You could fasten it up with some velcro, the kind with the sticky back would probably work great. You could make one big one or several small ones to vary the amount of ventilation.
Down here, ventilation is everything for preventing mold, mildew, algae and the like. We did this for a hatch for our boat, and it worked great.
