Cheers guys,
2F/2F, generally that's how I make the hole - slowly drill (with needle) sand drill a bit more sand ... until I get the right hole size.
It looks okay but under a mag glass the edges don't look clean. May be I am unnecessarilly meticulous.

Good idea using brass shimstock, Jim. Also aperture blade of an old lens could be good too. Aluminum is easy to work with but too soft. Thanks for good advices.

aaronmichael, thanks for the advice. My camera has a focal length of 155mm and according to Rayleigh's formula the optimum hole size is 0.56mm. Fortunately it has removable 'lens board' so I can test a few different size holes (lenses).