Lexan or polycarbonate is great stuff.
Check out release videos from past years. I remember Team 16 using polycarbonate in their arm in 2014.
https://www.youtube.com/watch?v=lcsi4zBwIjc
Hard to see, but at 12 seconds one can see their arm is clear.
A number of teams made their arms out of polycarbonate in 2016.
We made our arm supports out of 1/4" polycarbonate.
Its not as stiff as aluminum, but it's tough and bounces back.
Dave