Hm, sometimes I pinch thumb and forefinger together and then bring the opposing forefinger in to join them, forming a triangular aperture. Takes two hands, but I guess I like the level of control. :p

or squinting

That deforms the eye a bit, and the aperture is too wide. Doesn't work nearly as well as a pinhole made by other means.

I suspect the "aperture" is actually holes made with your eyelids top and bottom with your eyelashes on the sides.