Back in about 2000-ought-06 I did a pinhole lensboard for my old B&J Press 4x5 using soda can sidewall. It measured 0.004 inch thick. Of course, soda cans have gotten to where they need the soda inside to stand up, they may be thinner these days. (Lucky we can't see light through them! )

I then used 0.002 inch brass for the SQ-A body cap pinhole. And since have been using 0.001 inch brass which is admittedly a bit touchy to handle. I have also used chemical blackening on the last few (vs. black marker) and believe that is better. Some of those marker pens can leave small blobs here and there.