The present invention provides a chamber having at least one operator hand access port; the port comprising a port aperture located within the chamber wall; and at least two flexible non-resilient lightweight sealing gas-tight flaps (SFs), disposed the same plane of the port aperture in an overlapping manner, entirely enclosing the port aperture; each of the SFs is defined by a port aperture edge and chamber edges; the chamber edges are anchored to the chamber wall along more than half of the perimeter of the flap, at a distance from the center of the port greater than the radius of the port and beyond its perimeter; the port aperture edge of each flap is stretched across an entire chord of the port aperture; stretched port apertures edges define an interior access zone, disposed the same plane of the port aperture, characterized solely by the port aperture edges.