[QUOTE=Greg_E;540770]Aren't pie "tins" the "classic" pinhole material?[ /QUOTE]
Aluminum pie tins are what I have used. For blacking I use a sheet of black paper with a 1/16" hole backing up the pinhole.
The dimple and sand works the best because, as mentioned by Greybeard, it leaves a knife edge. I use a sharpening stone ['Moonstone, Case'] to sand the back. I glue the dimpled square of pie tin to the table and run the stone over it so it is as parallel to the aluminum as I can make it.
I put the pinhole in the enlarger for examination - make sure it is round, there are no burrs - and also to measure the size. Size is the hard part to control with the dimpling technique.
As there is little control of size I make several of several different sizes and then figure the optimum 'focal length' for each size.
I did the sand off the pinhole dimple trick with an aperture I made up from aluminum I salvages from the side of a standard north american soda pop can. The metal in the walls is as thin as I have ever seen. Then stick it in the enlarger like a negative, when the enlarger head is set to a known multiplication factor, like 10X, to figure out the effective aperture of your pinhole.
Go to your nearest dentist and ask for a couple strips of "dead soft matrix bands". These things cost cost next to nothing and couldn't be more perfect. They're stainless steel, and you can get six or seven holes out of one strip.
at risk of exposing how crude my methods are sometimes, here's my 2 cents:
I rarely do pinhole, but I often need a small f/22-ish aperture for cameras I make that use disposable camera lenses...I get a tiny nail red hot and burn a hole through black cardboard. Makes a nice round hole.
The sides of modern pop (soda) cans are very thin. The dimple with a pin and sand off trick works well for me. Then stick a known scale (paper can work here) negative in your enlarger with the head up high, and then project through the pinhole. Work out the length of the known length on the baseboard, then the measured length of the pinhole to work out what diameter you have made.