Something that makes a noise may help to keep them away.... our cats didn't really like the sound of tinfoil. However, cats do often like to crawl inside plastic bags so a plastic cover may not be the way to go.
They do say cat's stay away from a plastic bottle filled with water, if left lying in the garden (something to do with reflections but not sure how true it is). You could experiment by putting a couple on the roof when you leave it overnight and see if it works. At least it won't cost anything!
But if you've got a spare 9 minutes:
https://youtu.be/uIbkLjjlMV8