An enclosure for isolating material from the ambient atmosphere yet allowing free access to the interior. The front wall of the enclosure includes a window portion and at least one opening with a frame assembly mounted therein. The frame assembly includes a first movable access frame and a first flexible curtain; at least one second movable access frame and a second flexible curtain are mounted in the first frame. The first frame is movable from side to side across the opening; the second frame is movable from side to side across the first frame, perpendicular to the direction of movement of the first frame. As the first and second frames are moved, the first and second curtains move also, such that the only opening through the front wall is through an access port in the second frame. By moving the first and second frames, the user has ready access to substantially all the interior of the enclosure, and can position the access port wherever convenient for the task at hand. In a preferred embodiment, the front wall has at least two openings, each with a frame assembly mounted therein, one for each hand so that the respective access ports can be positioned independently of one another. Access to the interior of the enclosure is limited to the port(s) so that air flow requirements are significantly reduced without limiting accessibility. Reduced air flow results in reduced utility costs and reduced worker exposure to noxious fumes, toxic materials, etc.