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.