Wallpaper samples come to mind. They are (usually) free and small/light enough to carry and easy to slip in behind the subject. I have seen them in wood and stone patterns. Linolium(sp?) flooring cut to size or maybe even tile if you can handle the extra weight. If you (or someone you know) have left over formica from countertops, that could also provide decent patterns.
Or an enlarged photo of a pattern! Lightwieght and a DIY project.