I used 15mm round cabochons like these and painted the back with gold acrylic but there are other options! The smaller ones shown are 10mm. A small issue is making sure the eye domes and pupils glue into the eye holes without looking crosseyed or wall eyed! Larger pupils help solve this but then your vision is more obscured. Using no pupils with the visor material still looks nice and gives good vision but i personally like the friendly frantic look of the pupils.
I cut a ring of acrylic to make the glued eyes more finished. One could use stiff paper , epoxy clay, etc. It will push the eye farther away from the lid so consider this when choosing a material.
