A system for enclosing an instrument, module or other assembly in an explosion-proof housing. The system includes an upper housing portion that includes a first threaded portion and, optionally, a transparent window portion; a lower housing portion that has a second threaded portion that is structured and arranged to cooperate with the first threaded portion to provide a tight, air- and water-tight fit; and an inner mounting assembly for supporting an instrument, module, electrical circuit, electrical device, display device, or other assembly. In pertinent part, the lower housing portion includes integrated bosses that provide horizontal surfaces for supporting the inner mounting assembly and for releasably attaching the inner mounting assembly to the lower housing portion. The number of positioning of the bosses and the number and positioning of mounting studs on the inner mounting assembly are designed to mount the inner mounting assembly within the lower housing portion in a manner that is independent of the mounting orientation of the lower housing portion.