Many of ESI's installations were temporary construction covers, in service for a few months to a few years.
The Bethlehem Steel project in Pittsburgh was a large cover that was custom designed to fit around fixed equipment, overhead conveyors and catwalks. When erected, it fit neatly without touching any of the obstructions. Other than detailed design and care in installation, there was no additional expense incurred for this feature.
Very large covers, such as the environmental reclamation site covered shown here, can be manufactured off site and installed in a single piece. The weight of the cover is not a problem as blowing a bit of air under the cover during deployment assists in moving it.
Both small and large doors can be installed without airlocks. This is an important operating consideration where medium to high traffic is expected.