Take a belt sander belt and split it at the seam. Spray glue it to a flat piece of flat wood (in this case a 36 grit 3x18 belt to a piece of 3/4" melamine).
Forgot to mention that the cloth backed sanding belts will last 10X longer that sand paper. And, you can make a 36" x 4" sanding block if needed. Lots of different lengths and widths out there.
I also mount them to flexible 1/8" plywood for curved panels.