Just had another idea for moving air around yor greenhouse.
Lots of dead computers have 12v. fans in them that work fine, so why not fix one into a length of plastic drainpipe that will suck the hot air from the apex of the greenhouse, and blow it out at floor level. The fans can run off a car battery that is trickle charged by a solar panel, the type used to plug into your car cigarette lighter when you don't use the car that often. You could easily fix one system to suck air in from outside at floor level and blow it into the apex at the opposite end to the first system so you will cool the whole greenhose down, and both will run from a car battery.
Brian