How about laying the frosted tape on the plastic strip, then curving the piece of plastic around the film track with the frosted side ot the tape facing the lens? That would put it directly on the film plane. Also, the image would be right on the frosted surface rather that coming at the frosted surface from the back side.
That was how I was picturing it.
"There are two ways to avoid most trouble in life: live below your means... and within your seams."