You may just have to draft your own pattern. Study how it's pieced together, draft out on paper each of the pieces, and the sizes and add seam allowances (be generous). Cut out of a sample fabric (being generous with seam allowances) and baste the pieces together. I'd probably include some type of zipper so can can more easily get it off and on (and maybe even wash the cover), so leave an opening where you could put a zipper in the final product. Then use your sample to create a final cover.