I built last Oct. Didn't put a good support on the roof and it collapsed under snow, then I added two 2 by 4 to support it. I don't want to spend money on auto vent. And not sure if that works neither, I have a 2by2 window on one side, and a 2 by 6 door on another. Yesterday, the outside temp was 15 degrees, even I opened both window and door, the inside temp was still stayed at 35 degrees. I don't think a small a roof vent will help, and also it is hard to install on a PVC greenhouse.
Easiest approach in this case is to re-fit it so that the plastic sides can be rolled up halfway when it gets hot. This will give you good air-flow with the door and window, and cool it down somewhat. If it's too much hassle to retrofit it this way, get a piece of shade cloth and put it on top for sunny days.
