We're on our second matte box for the 3D rig and it's looking pretty ratty at the moment. I'm in the process of refining the design and building a new matte box. At the moment we're using heavy matte board and it holds up pretty well given the use it receives.
I'd like to see if I can source something like non-corrugated plasticard to make a stronger version in the near future. Things like this are always fluid and as long as we have a decent matte box for the glass we'll be good to go.