@EKT:
I mean extra plastic surface.
When you cut the top and bottom part of the shoulder armor, there should be a gap there when you offset both top and bottom plate that far isn't it?
Ouw. Right. I used a couple of unused shoulder pads to make the bottom ones. Since I'm using DX2's shoulder pads on the figure, the DX12's are free, and I butchered it. It's a bit hard to describe what I actually did, might do some instructions later. My method might not be the best way to do it.
Actually, I'll try describing it.
Obviously, you first make the separation (cut) the top and under plate of the stock shoulder pads apart.
Look at the picture you quoted. See the edge of the bottom plate? There' a cut near the top? That edge piece, you need to cut from the now separated bottom plate. Now you have a little edge piece of a shoulder plate.
Next, you need the shoulder pads of another batman body. That means, not the ones you're working on. Do the separation again, this time, take the top plate, and cut it in about half, keep the shape of the bottom edges intact, don't worry about the top edges. (This is on the second set of shoulder pads).
Now, you have 2 pieces to complete a bottom shoulder pad.
Glue the tiny edge piece onto the bottom edge part of the shoulder pad you cut from the second set.
Now you have 1 complete bottom plate. Use a double sided tape, or blu tack (what I use), and stick it on your Bat.
Phew... Not sure how clear that was...