Do you have enough money to run to petsmart and invest in an e-collar?
I know it looks pitiful but if the onesie idea didn't work then I can't think of anything to safely do to keep your cat from licking her belly.
If you have experience shaving cats, I would recommend attempting to shave the area down and apply a gauze bandage?
I also just read about someone who safety pinned a large sock around her chihuahua's neck to keep him from eating at stitches. Maybe that will work? Not too tightly, but just enough to keep her from craning her neck to lick at her tummy.