I'm building a buggy with a 2x4 square tube frame. Figure out the angles you need the frame to bend at (sketch it to scale on paper and stick a protractor on it, if you can't think of any other way), take your 2x4 tube (.120 wall, BTW) and cut a nice straight cut (I used a bandsaw) through the tube at HALF the angle. Then take your now two peices of tube, and flip one of them upside down. Weld. It's not really hard. It took me one day to build and fully weld both my frame rails (the full length of the buggy) with four bends each, and weld the two together with a couple crossmembers. Let me know if you have any questions or need anything clarified.
Here is a picture of what my frame looks like. Is that kinda what you're looking for?