i just put it flat on the ground. They mess around the inside and will perch on the sidewall of the tire. When it snows i run the shovel over the top of the tire to clean it off and the sun just melts it quickly. they have a warm soft perch. I used a 4 wheel tire, a lawnmower tire and two regular tires over the space of the run for them.
Love it! appreciate the picture! I don't have any small tires but i could easily purchase a small one.. great idea.