It's not anything I've ever used, but always heard they wored great.
If I was going to try to make them I think I would get 2 sizes of salmon egg molds. Shoot the small ones in the pupil color then drop one in each cavity of the larger size. Then shoot a clearer color on the outside. Not sure how well it would flow. It's posable the small ones could plug the hole between the 2 and cut off plastic flow.
It'd be worth a shot if that's a bait you use a lot.