If you're going for the Halo Wars 2 Mk IV, you're pretty dead on to what I'd do. The smaller shoulder plate is what slots into position with the outer bicep, and that *theoretically* means you can attach that plate to the arm there using some foam/plastic sheets and some glue. For the larger shoulder plate, mount it on TOP of another smaller one!
Not sure if this answers your question, but hopefully that helps a little. Send a picture of GA's arm files if you can