Cats like shallow boxes so they can get in and out of them easily. (cathealth.com)
They will need a small shallow box (you could even try the lid off of a shoe box) and only an inch or so of litter. (pethelpful.com)
These primarily earth - coloured paintings from the late 1950s and early 1960s were often textured with sand, hessian, metal and wood, and included collages in shallow boxes as they became increasingly sculptural. (englandgallery.com)