Yep, nylon and epoxy resin have similar indices of refraction, but it's not perfect. I tried casting thin rods out of epoxy to replace the nylon but failed to achieve a good result.

Have you tried Fluorocarbon? this has replaced Nylon for many fishing use (fly fishing in any case), it has a different refraction index -- not sure if it would be closer, or further from the resin, but at least it is different! :-)

Never tried, because fluorocarbon's index of refraction (1.42-ish) is further from epoxy resin (1.50-1.57) than nylon (1.53). It does make sense that fluorocarbon has replaced nylon because it's going to be less visible in water (1.33).

I guess the epoxy's index of refraction depends on all kinds of factors such as the mixing ratio and the conditions under which it cures.

Couldn't you make little sticks with the same epoxy resin to hold up all the parts?

I tried, but they're very difficult to cast. Epoxy resin is also very brittle and difficult to cut neat and square. A jagged edge does not bond very well to a smooth surface.