One way you could get out of the vetillation problem for less than using one of those Navy aproved box fans would be to rig a decent sized fan to an electric motor via a belt drive. I have a friend that has done this with great success. It was really cheap and simple to put up and take down. The fan was enclosed in a squirrel cage and had plywood sheets that where cut to fit to the door opening. The roll up door was rolled down to sit on top of the fan/plywood rig. The electric motor was outside of the space, and not in any contact with the atmosphere being pulled from the space, running the fan via belt drive. That rig moved a WHOLE LOT of air too. Where it sat on the floor, it gave a nice little downdraft too, and that is always a plus. Takes a little thinking and engineering to your needs, but it is an option. All you need is an old Electric motor. I am not sure where you are, but I would recommend an old poultry or dairy farm to get a motor from. My family had a poultry farm, and we had scads of motors laying around to run the ventilation in our barns. You could likely get the whole belt drive, fan and motor in one shot. Hope that idea helps.