Used mainly to inhibit access to an inspection chamber, non man entry caps or plugs are fabricated from sheet plastics, either polyethylene or polypropylene cut to size to suit the chamber. A hole is then cut to the required diameter.
This has the added benefit of allowing small covers to be used with larder chambers e.g. a 600mm chamber could use a 450mm cover and frame by using a non man entry cap to reduce the opening.
Made to order to suit customer requirements.