These are often used to contain antennas or other low-profile printed circuit boards. This style of enclosure is made up by sandwiching 2 or more layers of flats. The contour of these flats as well as the internal cavity are CNC machined so there is no forming involved. The stack is either solvent-welded or held together by fasteners (usually # 4 thread-forming screws).
This makes for an extremely rugged casing that can be dropped or stepped on without suffering any damage. This style enclosure is popular for USB-devices, RFID-tags, wands, antennas, key-fobs, remote controls and small sensors.