beth (defunct)

While not specifically for cats, my partner has now bought multiple heated mats for your back, some have been square shaped, and the cat has stolen each one.

I had an inkling, and indeed it looks like there's some shenzhen manufacturers that make USB-powered heated pet mats for exactly this purpose: https://www.aliexpress.us/item/3256804934280669.html

they supposedly stay at a constant 28 degrees celsius which I imagine is largely due to the fact that even a USB 2.0 port will only give you about five watts of power

this should get pretty warm (but it is a bit expensive)