How do you keep rats/mice, out of a compost pile?

I have a regular compost bin. It has 2 lids on top and one slide up door on the side to get the compost stuff out.How do you keep rats/mice, out of a compost pile?
This is not easy in my experience, but if you don't already have the compost bin on a solid base that would be worth doing when you move it, because they can get under the walls to get in.How do you keep rats/mice, out of a compost pile?
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