To avoid mold and casting in something clear or opaque you could print the parts in a translucent clear or a opaque semi transparent color (I use PLA). Then finish/paint like normal 3d printed items making sure to mask off all the areas you want to glow or light up. Put the LED's on the inside and you can adjust the light diffusing by where you place the LED's. I have used this technique with smaller props in the past.