I do a lot of scrappy quilting, so I would find fabrics that match lots of the different colors in your cat blocks, then I would sew up scrappy 4 patches to go all the way around. To make it rectangular rather than square, you can add 4 patches to the sides and add an extra row or two to the top and bottom. Then I'd made a final outer border from the sashing fabric, and perhaps bind it with that too.