You won't need to "make" square tubing out of angle, but when you put them in, position the angle where its laying down like a triangle ( /\ , not I_ or _I ) This will give you the lateral strength you need for bracing, since this is all you are trying to accomplish. (Round tubing can be used as well) This will also save you on material so you don't have to "double up" on the angle...
SVT