2007 we used kit chassis with LOTS of loose bolts (I wasn't around then, but many alumni have memories of the robot dropping things everywhere during matches and in the pits)
2008 was a little different. We used kit chassis rails that we bent around a bunch a bunch to make a cool looking but structurally weak octagonal frame with an indent in front for the trackball, and then bolted a bunch of 80/20 up off of that. The 80/20 was quite strong, and fortunately the chassis rail didn't have any problems.
2009 (yay for the basement!) we had kitbot chassis on the bottom and then two sheets of plywood up the sides. Everything inside the bot was then bolted to this plywood, with a nice plywood roof to top it off, making for a rather sturdy setup.
2010 we welded for the first time thanks to a local prototyping company who did it for free
. It was definitely the strongest we had ever made. We used the chassis for wheel mounting convenience and then aluminum box tube for a superstructure around that. Initially it was all riveted together, and then we sent it in to be welded, with gusset plates at the corners.
Attached are pictures from the years I could find.