Stopped at the discount hardware store and found a bunch of measuring tools on sale. This included a digital micrometer with an advertised 1 micrometer precision and a 4 micrometer accuracy for $35. I also picked up some feeler gauges. I need to build a smartphone science microscope next. Then I can image the slits and the feeler gauges or gap in the micrometer in order to measure the spacing and width of the double slit.