Still working on the sculpting of eyes, but if you're gonna use inserts, I recommend common craft store beads. The come in tons of sizes and colours so you can use them to scale. As well, they are pierced through the center for stringing so you can stick a couple on the ends of a small piece of wire for spacing and push the pair into the bridge of the nose area and align them as one before finishing.
Here's a quick example. I set the eyes early and they stayed nicely in place right till the end.
