Step 1: Get the camera
I chose the agfa clack, because it is robust, cheap and easy to modify. You can get this camera for a few bucks in online auctions.
If the housing is o.k., don't worry about the function. There is nearly nothing that can get broken.
Step 2: Get a sheet of aluminium
This is a very thin sheet of aluminium. You can get that from a soda-can, a tea candle or something similar.
Step 3: Make a hole
The perfect hole is done in multiple steps. Do not just punch through, but make a small dent and polish it away. Dent to the other side and polish again. Do it as long as it takes to make a small hole.
Use this calculator for the best hole-size:
I chose 0.3mm for the smalles hole, resulting in a aperture of 256.
I checked the size of the hole with a lead of a mechanical pencil, which is 0.3mm.
By the way: The makro-pictures were taken with my smartphone where I applied a laser-lens from an old CD-ROM-drive to the camera.
Step 4: Make another hole
I wanted to have aperture 64 and 16.
Therefore I needed one more pin-hole of 1.25mm and used the existing hole that was already there.
An aperture of 16 doesn't make sense if you want to remove the lens.
I also checked these with leads of mechanical pencils or other stuff I found around in the house. 1.25mm is aproximately the size of a rather thick pin.