As the picture shows, the 'white' of the iris is set back slightly,
so the rim casts a shadow, giving a better illusion of it being a 'seeing device'.

From top to bottom, the parts are...
Rim (black Perspex), eye cover (clear Perspex disc), bigger inner rim (black Perspex),
spacer ring (hidden but in black Perspex) and finally the white Perspex backing disc.

All the rims and discs were cut using a holesaw set, mounted in a drill press.
Where the correct size hole saw was not available, the part was made oversize and then sanded
down by mounting and spinning it in the drill press.