Hollow round tubing "probably" won't work? Given that you'll have a much easier time finding round tubing than square tubing or angle, wouldn't it make sense to first rule it definitively in or out?
You can readily get Delrin in rounds, round tube, squares, flats or sheets. Even hexes. Square tubing and angle may be available, but are certainly not common. You could, of course, mill the angle stock out of solid squares.
Maybe I should have said "it won't work as well" or "a round piece or solid square piece doesn't suit the application like an angled piece would".
Easy to find shapes are not completely ruled out, they just aren't best for the design. I could have gone into more detail about the contingency, but my main concern is that I haven't overlooked some place to buy angled pieces.
Might. If he needs 1" x 1/8" angle, milling from 1" square bar would yield pieces likely to warp, in addition to being extremely wasteful. On the other hand, milling 1/4" x 1/8" angle from 1/4" square bars would probably be fine.
Without knowing the size wanted, and knowing the lengths he plans to use, it's hard to say how much warping there would be. And whether the piece would be constrained by the rest of the structure, or whether warping would even be a problem.
He's obviously done some searching for the angle and square tube Delrin, without success. My only point was that, before giving up on the round tube and searching for something that may not even exist, he ought to definitively rule it in or out.