Just an idea I had,....... what about trying to get hold of a fridge like the ones all the neighbourhood shops use to sell vitagen and yakult. They are glass (see through for adequate natural light) yet should keep your plants cool but not freezing. I have no idea if it is practical or where you could obtain one unless you call up vitagen/yakult and agree to buy a years supply of their drinks!! Just an idea!!
Happy growing, Neil.