Step 8: Optional Anti-Slip Pad
Depending on the type of grain and finish your leather has it could slide a bit. Mine did so I hunted through my storage and found an old laptop case's padded, rubberized shoulder strap. I'm sure you can find something similar in a thrift shop if you can't cannibalize an already owned backpack or case. I've owned and seen many such straps that are either completely made of rubber, or are textile with rubberized pebbling on one side.
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.