We built a set-up that looked a lot like the drawing you have, out of treated lumber.  It worked OK, but wasn't a perfect set-up.  Just think about all the corners, that was the source of any problems we had.  Livestock loves to go to corners, the more you have, the harder it is to load.  The wide area before the loading area could also be a problem, our livestock wanted to turn when crowded.

Don't get me wrong, our set-up worked great compared to how we used to use the whole barn, using baling wire to tie panels together and tie the portable shute to the truck.  My only comment is to point out some potential problems compared to the tub loaders that force livestock to a narrow place where they cannot turn around.  If I were in your position, and already had the materials you describe, I would build it and be happy.  We didn't think we could afford those fancy tub loading areas either, but we could dream.  A lot depends on how well you have culled your herds for wild animals, that was a constant job when we ran livestock, but we seemed to often have some crazy individuals to load.