Step 8: Optional Anti-Slip Pad
The strap I used was about 9" inches long with a 2" slot that the belt goes through. The slot, of course, must be wide enough to feed the belt through. For obvious reasons, you add the last two suction cups after sliding on the pad. Make sure the rubber is opposite the suction cup side (though you'd figure that out).
The rubberizing really makes a big difference, keeping the instrument very stable.