Abstract: A five sided steel box with a honeycomb member therein formed by corrugated sheets and defining a plurality of hexagonal elongate recesses. Spacers on the outer faces of the walls of the box have tie bars secured transversely thereto and the ends of the bars are welded to protective hoops. A door closes and seals the box and comprises a framework, a panel mounted on the framework by means of clamping screws or parallelogram linkage and coil springs, and a seal between the panel and the open side of the box. The framework is preferably hinged to protective hoops of the box at one side and provided with a latch arrangement at the other.