Steve's point is spot on. Last time I made a pinhole, I used very thin copper sheet, in which you can make very nice pinholes and should be able to go very wide. I don't recall the thickness of copper I used, but it was like a foil... which is of course a bit flimsy, so I just cut a hole out of a more sturdy material (delrin) and taped the foil on there and voila.