As per the rules you may use Angle, box or round Alum.
You may NOT use speciality alum. extrusions. This includes 80/20 Alum, Bosch or the extrusions that can be bolted together. Several teams in FIRST used this type last year.
The intent is to use materials found at the local Home Depot or Lowes. Not materials ordered form speciality companies.
If you are not sure the Alum you are using fits our definition, please send me a picture (cross section). better to be safe than sorry.
You’ve said we can use standard stock angle, box or round Alum.
Can we also use the standard stock Aluminum “C” or “U” channel available at the hardware store?
It’s a lot cheaper than drilling and bolting together a bunch of angle material of different sizes, or wasting valuable time with a drill, Dremel, and a file on a box extrusion to do the same thing. (We’re rather short on staff, AND money, you see… 
Thanks!
C channel (a.k.a. U channel) is officially now legal.
The intent is to make it easier for folks to make a robot without getting too crazy on costs. C channel is available at most hardware stores.
Don’t press this rule too far. We are trying to limit the materials to “plain jane” stuff. Please comply