I plan to make my next insert out of this:http://www.amazon.com/gp/product/B00JPHT...ailpage_o04_s00
It might be over kill but I will be hanging two graphs from it.
HDPE is very strong and resists stress cracking. It's also cheap and easy to work with.
Aluminum is a good choice but it needs to be aircraft grade (hardened). Its a little more difficult to work with and tougher on tools.
Carbon fiber is good but is expensive. It is very strong and very light. Probably need at least a 3mm thick panel, 4mm even better. It is easy on tools and easy to shape and looks cool.
Acrylic is not good for this application. It is brittle and can shatter.
ABS is OK. It can develop stress cracks over time and fail with heavy loads. It has a low melting temp so low rpm tools to get the best results but it also means you can shape it somewhat with a heat gun.
Lexan is a good product but more expensive than other alternatives.