As you see on the bottom picture, interference artefact is not that visible on reflected light at 200 mm distance, but I assume that dot appear bigger because of that. The picture where interference shows is the same beam, but at three metres trough matte glass. When I viewed at laser side the effect was not very clear just a bigger dot (about 2-3 cm). But at the other side the effect is very clear. Last picture shows the interference nicely (Picture is taken at a angle, because head on the picture was just just a big blow of red light, but vhen observed the rings were very consentric and all around the central dot). But the dot was really round, only the interference rings to make it too large.
I'm testing the prism, because I want to slip the indicator at the end of the spindle and there is space only to very thin object. Therefore my preference is to make this flat, but if that is too much trouble or the smaller laser is not small enough, then "Plan B" is to fabricate one that will slip over dedicated ER holder - not as versatile, but plenty of headroom.
I don't know much of the pinholes, but I think that thinner the material you have the better the results are. I could not find my very thin copper foil, but I should have somewhere a little of thin brass shim stock and I was thinking of comparing that with domestic aluminium foil - hole pricked with a needle.
Pekka