I take the foam and turn it inside out. Then take some scisors and cut a 1/4 inch 45 degree angle bead out of the edge or each side. Try to keep the cutting bead consistant around the entire foam dognut. Then turn the foam back normal and it the side walls will have an inward slant to them. \ /. This will help it to fit better into the rim and better into the tire as well.
TQ Racing Wire