This is both one of the things I'm most excited about and one of the things I'm planning to build. I scratched the idea of buying a fixed rear window flare because they are bulky, add limited space, are expensive, and don't look tight.
I found this video of a guy (Dirt Lifestyle) that made an amazing aluminum tilt-up gullwing window.
My addition will be 2 parts:
1 - Add small glass sliders inside the aluminum wing.
2 -Make boxes that can fit under the wings and against the van sided when the wings are up. This will give a massive 18"+ inch extension - per side! More than needed for full stretch side sleeping.