I was tinkering with the idea of creating a custom canopy/window for eldar mechs.
Depends a lot on the shape and level of detail you're after.
At a basic level you could use the clear plastic GW use for their blister packs, it's easy to work with and best of all free.
Something like Woodland Scenic Water Effect (Not to be confused with their Realistic Water) could give you some options. It's like toothpaste out of the bottle and sets clear while staying fairly flexable. However I'm not sure how you'd get on trying to get a smooth finish on it.
Top end would be a clear resin, it will get you the best results. But... First you need to sculpt a master, then make a 2 part mold (Not hard, plenty of guides on YouTube but can be pricey getting all the equipment required) and lastly you'll need to be able to cast with zero air bubbles which again raises the costs for equipment.