The arches are just part of a facade that goes on the arm, and the basic shape it just a little rectangle and a big rectangle. I suggest buying/trading for just the arch bit (or the whole right shoulder), and carefully cut vertically the 2 rectangles apart. Then glue/
GS the smaller rectangle to the right side of the bigger one.
Now it depends on what you want to do about the actual rocket launcher. I'd buy a standard Dread Missile Launcher arm and add the arches bit, and little plasticard/
GS to mimic the rest of the trim. The only detail you're missing in the inward curve at the top of the Ven. Dread shoulder. If expense (with in reason) it no problem, I'd buy (or covert you current) VD left shoulder, and use the top curved part. Really if you buy the regular Dread
ML, Arch bit and VD left shoulder there are a bunch of ways you can do it, just depends on what you think is the easiest method.