We have always used angle aluminum. We have used anything between 1.25" to 1.5".
It tends to hold up pretty well. We occasionally get a few dents/bends. Nothing that cannot be easily fixed.
Our 2003 machine (rectangle) was probably our strongest. Even 25 couldn't dent us. We dented them....

The only thing that could have dented our 2003 machine was the shippers.... (Long story)
It was at the Rutgers Regional and on pratice day our bot ended up arriving halfway into the day. Then we open up our crate and notice everything was messed up. Controls broken, batteries all over, tools everywhere, and pieces of the robot inside. Practice day was gone... Eventually we got everything working for friday. The must have dropped it or flipped it some how.
