I'm playing with a couple of options right now - the one in the pic is simply some black plastic that's about card thickness punched at 2mm. It's then slid under the etched metal "screen" and sits gently wedged between the screen and the translucent plastic eye part. It doesn't move, so no need to glue and it's reversible.
What's interesting is that with the lights on, light blurs around the new pupils a bit, creating the illusion they are smaller. So they look just a touch too large in the pic, but in-hand they look about right. So not sure which one to go with - in-hand correct or in-pics correct.
I painted the upper lip weathering because it's part of what's making the head look so roundish. Painting that bit adds more gold and elongates the face just a tiny bit.
As you can see, the spikes just pop out of the holes on the neck and can be turned a bit. I may fill those holes with a small gold plug but they aren't really visible at normal viewing distance.