Lest you think I was drunk when I sewed it, the liner itself had a noticeable bias which made it impossible to match corners and still have the liner lay flat.
A while ago we heard the cat upstairs in the craft room fighting with a plastic bag that I had left on the floor. He likes the crinkle, so I inserted it into the mat:
And now he has a super-plush non-skid crinkley mat to knead:
I leave it in a chair he likes and he mostly just sleeps on it.