Brick brings up a very important point.
Once the box is on it adds a lot of stregnth. You create what is known as a "Box Beam". If you build a 3/4" solid wall, and bolt it to the side of the frame, you have made it very difficult if not impossible for that frame to flex.
Why, because you have installed 2, 4'X 3/4" beams running front to back.
Same thing with a stick built wall bolting it to the top of the frame.
This frame,
is built out of 1 1/2X 1 1/2X 1/4 angle.
I've built another one out of "Bed Rail" (6')
This frame has both the outlookers made from angle and square stock.
You have to look at your build as an entire unit. Its Not just a frame.
2X2 angle is fine for up to about 1200.bs (I think more on a Tear) as per my father in law the Engineer. But he thought I was just making a frame.
Free Engineers have no imagination.
Happy Trails
Len