If you don't mind, describe your setup, how much you need to raise your rear bag, and why.
The donut is a rectangular spacer that is made of leather and gives extra height and stability by virtue of having a tube, that the user fills with sand, around its outside edge. The central portion, inside the sand filled tube is not filled. Sinclair sells them. They were devised and first manufactured ( I think) by the late great Skip Otto, and are also referred to as "Otto rings". Because the rules disallow spacers that contain the rear bag, but allow multiple bags, his design cleverly skirts the issue by being a sand bag that just happens to have an advantageous shape. It allows the bottom of hard bottom bags to conform to irregular surfaces, and greatly adds to the stability of soft bottomed bags, while raising both.