I'm not a metallurgical engineer by any stretch bit I'm guessin' a van roof rack might have a little more flex than you want...
Seriously though, we're all on budgets of one type or another but there's an old adage about race cars that applies to go karts:
You can be cheap, you can be fast, you can be safe. Pick two.
You'll probably get enough advice in this thread on how to do what you want as reasonably as possible, but somethings you may have to spend some $ on or do some scavenging and obtain in order to keep out of the Emergency room.