[FrankJ]

1) 80/20 I know you said no t-slots but it is so useful especially for rookie teams & prototyping. You can get a deep discount if you tell your distributor you are a First team when you order it.
2) The Andymark C channel. reference holes are nice
3)The VEX 2x4x.1 rect. tube reference holes & lines are nice.
4)1x1x.125 angle. easier to properly torque nuts than in tube.
5)1x1x.125 square tube. About the same weight as 1" 80/20. Slightly stiffer.
6) 1/2 hex. Great for shafting