Personally, I wouldn't bother covering a compost bin with the intent of keeping out raccoons. I've rehabbed hundreds of the little critters, and I can tell you that their nimble little hands can defeat anything, short of a chain with a keyed or combination lock. I've even had them untie complex knots, open dog-leash swivels, and undo the carabiners that screw together. And they can squeeze through a space that you'd never imagine they'd fit through.
I remember one big guy that I had, who had been confiscated from some people who moved up from California. He weighed 65 lbs, and he didn't like being caged. For one thing, he had been raised as a pet and was afraid of the other raccoons in the cage. (Part of my job was to try to make him wild again, so he wouldn't have to be destroyed.
I finally managed to catch him in the act of getting out of the chain link enclosure. The roof, which was also chain link, was attached to the top bars of the sides with ties 6" apart. That raccoon was squeezing out through those 6" spaces!
"Isn't it enough to see that a garden is beautiful without having to believe that there are fairies at the bottom of it too?" - Douglas Adams