A project that has little to do with LEDs acutally, haha...
You seem to have us guessing as to the end use but sounds like light diffuser of some type...but not LEDs?
How well this works optically if the source is behind the lens v. molded inside is a good question.
The idea of making an optical diffuser (or whatever) with casting resin is intriguing...I worked with this material years ago and had lots
of fun embedding coins, small crystals etc. or just making unusual shapes. Point is, usually needs machining afterwords..
grinding/sanding and polishing/buffing (reminding me of waterproof 400-grit sandpaper and buffing wheel).
Dave