Vinyl flooring is what I've used before, for a bathroom or kitchen. Just have enough poking out the boot to cover what needs covering.
My mum used carpet, not to protect the paint but to provide purchase for their paws. It protected the paint too.
You can get thicker self adhesive vinyl from ppl that make signs tho
I have a fold out boot liner, works well. I looked into PPF and it just doesnt seem worth it. Its expensive, to the point that a respray isnt much more, and it can actually damage the paint when removed if left on for years.