U shape is probably best.
I think moon shadow has the right idea here...
it sounds like you're thinking of building permanent planting tables.. basically a raised bed on legs, right? Tell us more about the purpose of the greenhouse. Will it change seasonally?
As for ventilation, there are passive methods, based on vents and convection currents that cool, that work based not on absolute temp, but the difference between internal and external temperature. some semi passive, like hinges that open based on passive expansion (temperature) of wax or fluid inside to open vents at a specific temperature.
Fans and other electric equipment, sensors (wired or solar) give you the most control, but they are also the most expensive. Do some research to figure out what will fit your design, materials, budget and lifestyle
