I've watched this used on one of the restoration shows on DIY and the result was amazing. Cost isn't so bad, particularly if you need to make more than one. You can do it very high tech, but the guy on DIY did it in a plastic container with basic tools from the garage/kitchen.
You make a silicone mold of your exisiting lense, then simply mix the Alumilite clear (or white, or add colors like amber or red for maker lenses) then poor in and press the mold back together, works very well from what I've seen.
They have starter kits available that have all the materials for small jobs.
Alumilite - Products