The chassis for the robot Peter’s been working on is made of aluminum extrusions called Makeblocks. They’ve worked out very well. They’re robust and flexible. In addition, the holes line up with the holes in Lego Technics, so you can connect all sorts of things to them.
The people who make them have a Kickstarter project going. It’s a great way to support their work, and they will be sending some great gear to backers. We’ve supported their project, perhaps you should consider it too.