Sewing Pattern for a Cover for a Dog Bed

I am looking for a sewing pattern design that can be used to make a form fitting cover to fit a small dog bed. The material should be able to stretch in order to fit a range of sizes in the small dog bed category. The design is for rectangular dog beds with 3 or 4 raised sides only and not for square or round beds. The most common size in the category of small dog beds is 22 inches (length) x 18 inches (width) x 6.5 inches (outer wall height). The bottom of the cover will most likely be a velcro or stretch opening. This universal small bed cover needs to fit a bed as small as 20x16x4.5 and as large as 24x19x7.5 with the most common size being 22x18x6.5. The material will likely be a stretchable material. I included pictures of a very common small dog bed that the pattern will be used for a cover for this type of dog bed.