Great project. I'd recommend moving the tape measure a bit closer to the center of the field of view, because some of these lenses can have field curvature near the edge, meaning if you calibrate focus on the edge, the center may be a bit off.
From my pinhole camera experience, anything shiny, black or not, will reflect bright sun. Craft stores have adhesive black felt, in sheets, that works good as flocking.
Keep us update on your progress. Well done.