If you can do a basic ostomy pouch cover (I do believe this would be easy enough), but leave a small hole on each side to allow for a standard accessory belt to be clipped onto your appliance, then it should work well to both support and conceal your appliance.

Of course, true vertical ostomy belts will apply some pressure to the appliance to lower your profile, but those are more complex to make and would require a more specialized fabric and a belt that's attached directly to the pocket that holds your bag. 

If you do make something, I'd love to see it!