I didn't want to paint my model (sanding, priming, blah blah, boring) so I made a couple of small black discs to insert into the eye sockets.
They're 0.4mm high so just print with 2 layers at 0.2mm.
Mine fit perfectly with friction, but you may need to scale up/down depending on your printer. Also, a dab of glue wouldn't hurt.
Hollow_eye.stl | 49.9KB |