[fellerts]

I never gave that any serious thought. What additives would do that, do you think?

Edit: A quick search reveals that there is quite a lot of prior art from the optical community. Darn, maybe I'm not done here.

[mensetmanusman]

You can use index matching fluid to determine what the nylon line is currently at and then use the fluid to experiment with epoxy droplets containing various amounts of low index additives like MgF2 nanoparticles or something :)

[fellerts]

My drawer is already full of radium dials. I'm not sure adding nanoparticles to the mix is a very good idea.

I've never heard of index matching fluids! Will have to read up on this.

[encom]

The rabbit hole never ends. Godspeed, friend.

[PicassoCTs]

Add a polarization filter coating to the epoxy? https://en.wikipedia.org/wiki/Optical_filter#Polarizer

Also, if the plastic can be "invisible", you can also house it in invisible plastic- thus creating, a exploded working model, inside the resin