Look for local sources for affordable pricing of 1x1 and 1x2 aluminum tubing. If you can find a local source who is willing to donate material, that is even better. If not, you can find great deals at
www.onlinemetals.com
Tubing can be connected using gussets or by welding. A great example of the gusset method is Gus team 228 (no pun intended)
http://www.team228.org/media/pictures/view/4896
Check out the way they connect their corners using what I think is 1/16" thick 6061 aluminum sheet. I personally love this method because it is easy for teams to do without much (if any) precision machining. You can CAD up a design for your brackets, print out 1:1 scale engineering drawings, and just use the print out as templates for pieces to cut on the bandsaw and make a hole pattern with a drill press. A lot of teams use this method along with rivets but you can also use screws and bolts. Don't use something bulky like 1/4-20 though. Rivets are recommended.
Hope that helps!