After reading your explanation, I would have bet money that you were a teacher! Please forgive my attempt to explain diffraction to you! I should have read more carefully. Also, the idea that narrower slits would spread the higher order spots more makes sense. Perhaps the 1/150 to 1/200 FL "rule" is just a practical compromise. As for materials, I have made a few masks out of Formica, cutting the pattern with a router. It is not too hard to set up a jig to cut nice straight lines at the proper angles. Thanks again.