I use regular thin aluminium foil for the hole, because it's thin enough to start with and you don't need to do the trick with denting it in and using very finse sandpaper to prepare it. It's not as robust, but good enough if you're careful. The trick is not to push the needle through, but put the foil onto a piece of cardboard, just lightly set the needle onto it and twist is while applying very little pressure and then shave away the edge with a very sharp razor. I can consistently get holes down to 0,2mm with almost perfectly round shape like that. I usually just do 10 holes at a time and see afterwards, which one fits best for my camera by scanning them digitally and getting the exact size that way.
I don't care too much for the optimal size, because the difference is almost invisible anyway. I just try to find a hole that gives me decent exposure times like 1-2sec in bright sunlight with 100ASA film.
Using a piece of high resolution, high contrast film, photographing a tiny black dot on a white wall and using that as pinhole might be even better, though. I know it is used for zone plates or pinhole sieves, but I don't see, why it shouldn't work for single pinholes. It's probably the easiest to way to get exactly the right size, because it can be calculated beforehand.
The thickness of the pinhole material only becomes important, when you want shorter than normal focal length, because thick materials show much more vignetting from blocking the light with the edge. Some unprocessed metal from a soda can might get you an image circle of about 50° without too much light loss, aluminium foil gets you 100° while filed downs brass sheet gives 120° (these are no real or tested numbers, just an example). So if you're not using some ridicilous wide-angle like 30mm on 4x5, it's not that important.
Feel free to criticize me for that - I'm a practical pinhole photographer, not a scientist or someone who examines every picture with a loupe.