I would cut strips from each of your fabrics in varying widths if you are making anything larger than 45 inches wide you will need to join strips to get enough width. I am not sure how many fabrics you plan to use, but you could cut a 2 1/2 and a 4 or 5 inch strip from each fabric and then seam them together randomly and if you find this hard to do simply place all the strips into a bag and the next one you pull out is the one you attach (unless it happens to be the same as your last strip).
Good luck and have fun with this, I really like the idea.