Oreo likes the beds that are called "bolster" beds because she likes to have a pillow for her head. They have a 4" thick flat bottom and then a soft round bolster that goes around the edge. I bought hers online at Drs. Foster and Smith: http://www.drsfostersmith.com/produc...3&pcatid=14153

If those are too expensive, you could try this: You could buy a thick piece of "egg crate" foam or "orthopedic" foam in the size you need from an upholstery fabric store or an upholstery supply store. Then, you can sew whatever kind of cover you want for it.