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I built a frame out of one-inch rolled hollow section square steel and lined it with timber, leaving slots to enable tying the bale.
The ribs in turn support a gridwork of smaller steel frames, constructed out of hot rolled hollow sections, cut and welded at complex angles.
A fan assisted system draws fresh air from holes around the timber props, heats it and circulates it along the two main rolled hollow sections of the floor, whence it is distributed via perforations in the steel and the edge section of the floor up the inside of the glass walls; extract is through holes in the middle of the floor.