I cut a large hole in a lensboard and then glued an old filter step-ring to it (in my case, leaving a 49mm thread). I then knocked the glass out of an old 49mm filter and replaced it with a piece of aluminium from a drinks can with a pinhole poked into it. I can then screw on the pinhole in its 49mm holder to the lensboard. I have made 49mm rings with a range of pinhole sizes from 0.2mm to 0.5mm and also one with a 3mm hole so that I get a fuzzy but bright image for composing the scene.

Best regards,