[Sachiel7]

Plain old wood. It's actually a 1" square extrusion (non hollow) bar.
As for the riveting, we thought of that, but we mainly need the welding for the crossbars, thus strength is the key. But a good suggestion, none the less.
One side looks kinda like this:

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//\\//\\//\\//\\//\\//\\//\\//
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Your typical crossbar pattern.
Since there's no 45 aluminum angle, you can't bolt it (easily), and it would take lost of bolts just for one side. I think we can dig up someone who can weld for us. My main concern is if a weld breaks at competition, then what? Hopefully another team would be willing to help...