Take a piece of cardboard the size of the bulletin board that you want and cover it with cloth. To be extremely frugal, use a piece of an old sheet or other cast off fabric. You can have it as fancy or as plain as you like. Wrap the cloth at the corners and fasten with staples or thumbtacks and there you have it. You can staple or thumbtack a string to hang it or make a small hole in the top center for a nail.