Simply unscrew the lens cap off one eye-piece, remove the lenses, and cut a 5 - 6 mm slot 90 degrees around next to the flanged end (or, if you're wanting use the lenses still, make allowances for their thickness) - a Dremel is best for this job, but a junior hacksaw will suffice. Insert the iris into the cap, with the handle protruding through the slot. Screw the cap back onto the goggles with the iris inside, and adjust the position of the iris so that the handle will fully operate the aperture. Finally, fully tighten the lens cap to hold the iris in place.
Blades are printed flat and raised with supports. Suggest printing them all at once plus some spares.
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