We have used water-jetted aluminum plate for our frames, with other assorted aluminum and polycarbonate machined pieces for mountings and other parts.
Before that we used 1" welded aluminum box. We stopped using that system because it tended to be heavy and it's hard to modify if you need to change something.
The water-jetted parts are perfect and light but do put a lot of pressure on the CAD operator not to screw things up.
