I’ve found that the hardest thing about my small greenhouse is keeping it from getting too hot inside. Even with the end door open, it is often 80 – 90 F inside on a sunny day, which is way too hot for the seedlings I am trying to raise in there. I made a simple vent out of a couple of scrap 3/4 inch pine boards that I sawed into narrow strips and screwed together along with a spare piece of greenhouse film.
Basically, it is made of just two rectangular frames, one frame an inch bigger on both sides so it can fit around the smaller one. The bigger frame is covered in plastic.
The small frame was screwed down to the greenhouse frames on top of the plastic film. The film was then cut, wrapped around the sides of the frame and stapled.
The bigger frame just sits on top of the smaller one. There are no hinges. I just lift it off and slide it in through the opening on sunny days. I will probably need to hold it down somehow, maybe with a bungee, but that hasn’t happened yet.