Use an overmat with a window showing the dirty border plus some of the unexposed paper. Take a piece of mat board and cut two "L's" you can move them around the print to see the effect and how much extra paper you want to show. Measure the amount so you will know what size window is best and for that matter if you want to change your mind. Keep the "L's" to use when the occasion arises again. This way you won't waste boards by cutting the wrong size window or how you want to present a print.