A front cover is provided with an open area to accommodate visual displays, keyboards, switches, joy sticks and the like. A back cover, nestingly interfitting therewith, is closed, but provided with a recess, encompassing most of its back area. One or more identical nesting interfitting intermediate frames may be nested and stacked between the back cover and the front cover to form a package of any desired volume. The frames, back cover, and front cover are provided with means for mounting functional elements such as electrical circuit boards. The front cover and intermediate frames are provided with peripheral circumferential ribs. These ribs, and the slots therebetween, provide for a great variety of ways to mount the package, as does the rear recess in the back plate. The front cover may be identical or nearly identical to the intermediate frames. Functional elements may be mounted to the covers and frames during manufacture such that the covers and frames act as protective carriers during manufacture. After assembly, they provide a close package or case for the assembled unit.