Laminate a sheet of A4, Letter size, or 9" x 12" paper. There are 3 ways to do this:
Use any dark-colored foam board or poster board. Cut it so that it is at least 1 inch bigger than your laminated paper on all sides. You can also match the foam board or poster board you buy with that of your paper so that you won't need to cut it. For instance, if you used 9" x 12" paper, then an 11" x 14" foam board or poster board would be the perfect-sized backing.