A while back I made a mattress for one of my cats. The outer fabric is one that I really didn't want to use in any quilts so it was a perfect choice. For the stuffing I filled it with fabric trimmings, batting bits, and even some small pieces of yarn. To add a little height, I boxed the corners.
It was a big hit with the kitty.
Next I made one for her carrier. Sometimes she goes on long trips and needs to be comfy.
Then I made one for my other kitty. This time I used some string blocks found in my UFO bin.
The thing I love about these mattresses is that they are made of materials that I would have thrown away so if they get soiled and don't come through the wash well, I can toss them and make some more.