said by Ken:I still don't understand why you would need an insulated cover.
It's not there for it's thermal insulation capacity, it's there for it's sound insulating capacity. It makes a noisy compressor quieter. As long as the compressor is properly serviced and in good working order, the retention of the heat by the insulation should not impact the compressor's operation.
I'd clean out the condenser cabinet, then seal up any openings that are not screened over that would allow mice to enter. They are entering the cabinet because it's warmer and/or more protected from the elements then alternatives.
The other option is to put a cat in there. Depending on the frequency of the mice visiting, the cat may or may not make it very long so you might be trading one problem for another.