I don't know much about coldframes, but I'm thinking that if the shower door is hinged to the frame on the back then the hinges should assume part of the weight of the door when the vent-opener opens. If that's the case, then the vent-opener wouldn't have to lift the full weight of the door and you could use a less expensive model.
I bet the people on our Greenhouse Forum would be able to help you out with the physics and calculations.
http://garden.org/forums/view/...
