It worked
The outdoor cats have been using it for years, and the cover has torn a bit from all the washing, but it was easy enough to sew back together. They love lying on it, and in the late afternoon I warm it up for them; if they sleep on it all night, it’s still warm in the morning. In my opinion, it’s the most reliable bed warmer. I wouldn’t sleep easy with an electric bed warmer, to be honest. This one can be used even when they’re not being supervised.