From memory, without nipping out to the Garage at this time of night, the front part of the box is supported at the bottom, by two clips that fit, (poorly!) onto two tags welded onto the vertical frame tubes, about six or seven inches below the tank supporting rubber domes, (the ones that push into the vertical frame tubes behind the carb's)
The top part of the box has a hook that uses a large O ring, (Hoover band?) to hook over two thin rods welded either side of the central top frame tube, in-between the aforementioned tank supporting rubber domes.
This helps hold both halves of the box together by backward pressure of the whole thing onto the frame tubes, as well as the inlet rubbers.
This may be overkill, but if memory serves me correctly, it helps hold the airbox in place/together while you wiggle the inlet rubbers into place, (if you are using the internal plate and damping rubbers, utilized for EU Type approval/noise considerations I suppose, "Heaven Forbid!" then it weighs considerably more, so it provides additional and maybe unnecessary extra support)
