Can anyone recommend a good cover for a 2015 Prius?

You might be better off to deal with the cat and the result of same.......if any.
An improper cover can damage the finish as the wind blows it around.

 

I had three cars, one of which I had to keep outside. Come winter when the snow and ice and winds came, I bought a custom made car cover of Noah fabric from calcarcover.com for my Porsche Boxster convertible. Soft inside, tough outside, breathable. There were many days I scraped 8" of snow off the car cover and removed it to take that car out and both the car finish and the cover were in perfect shape 5 years later when I sold both.

Consider where you'll put it to dry once it comes off.

The cover was held on by elastic. You could also get a wire and lock which would attach to grommets in the cover but I never used mine. Folds up into a cloth sack maybe 2 feet by 3 feet.

They do have a cover for the '15 Prius, even one with a pocket for the antenna. I'd think that a real pain to fit and a risk to the antenna.

Never used one for a Prius sized car.

Not cheap but worth it.


California Car Cover Company - Car Cover Materials

 

You misunderstand.
It is HIGHLY unlikely that the cat will do any significant or permanent damage.......if you just leave it alone.
If startled while sleeping up there some claw marks might show up but it will mostly be dirty paw prints to clean off.

If you really must do this, don't cheap out on the cover.
Check out the one recommended above.

 

here is the one in my Amazon Wish List.