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I made one that uses 5x7 photopaper and has a "subject distance" between 1/2 and 3/4 of an inch... but it works quite well down to 1/4 of an inch or even less.
Not sure what the magnification factor is, but a photo of small pocket watch did not fit on the paper... maybe 3 or 4:1, and had lots of detail.
It uses a pinhole that is a bit less than 0.15mm but it is very difficult to light the subject well. I haven't used it very much because of the difficulty with lighting.
I've had two ideas along these lines that I've never tried....one is to mount a small platform at the pinhole to put subject on..with a flexible little book reading lamp mounted on each side for lighting..if the platform was made from a microscope slide it could help with illumination. I was imagining a small flat black background for the mini "stage". An advantage of this is that once you've figured out the exposure, it could be used for lots of tiny things ( I was thinking of a bee's face or a beetle's face... but never pulled it off )
The other idea is to make the camera pyramidal, with the pinhole at the apex.. but when I worked out the angles, it still seems like it would be difficult to get light on the subject, just less difficult.
Have fun, it's definitely possible!
Edit: the "focal length" on mine is 4" so I guess that would be a magnification factor of about 8:1.
Here's a photo of some hypo "prismatic rice"... I put them on some glass above the pinhole and lit it from above....