Two lengths of 20x20mm aluminum extrusion were provided in the 2014 kit of parts.  Also included were some thread-rolling screws for use with the extrusion.  Catalog page:  http://www13.boschrexroth-us.com/Framing_Shop/pdf/101/Page%202-11.pdf


Parts given as part of KOP 2014:
  * 3842992888 - two pieces.  20x20mm profile, 1.5m long http://www13.boschrexroth-us.com/Framing_Shop/Product/View_Product.aspx?category=10101&subcategory=1
  * 3842528593 - S6x16 self-tapping connection screw, T25 Torx head 
  
the self-tappng screws are used to make a fixed right-angle joint, as shown in the picture on this page:
http://www13.boschrexroth-us.com/Framing_Shop/pdf/102/Page%203-17.pdf

===== Additional material purchased =====

  * 3842517173 - more of the same 20x20 aluminum extrusion, with unfinished ends.  3m long.
  * 3842528593 - more of the same self-tapping screws
  * 3842523511 - gusset for adjustable right-angle joint without drilling. with hardware:

the gussets are shown here: http://www13.boschrexroth-us.com/Framing_Shop/pdf/102/Page%203-4.pdf
and here: http://www13.boschrexroth-us.com/Framing_Shop/Product/View_Product.aspx?category=10201&subcategory=1

===== Other Compatible Extrusion =====

Lots of places have compatible 20x20mm extrusion.
  * "8020" brand
  * https://www.adafruit.com/category/122 - we bought some bolts and t-nuts here


===== Summary =====

there are two ways to join this aluminum.  drill and use the self-tapping screws in the corners of frames (fixed structure and catapult).  use the gussets if the position may need to be adjusted.
Use both if a corner needs to be especially strong.