I wanted all of my Crimson Fists to have raised squad shoulder pads and the price for some custom ones online was pretty prohibitive if you want 10-20. I couldn't find any of the old metal GW ones either. After actually grabbing a devestator I remembered that the shoulder is part of the arm in most cases. Not wanting to spend $4 a shoulder plus make a lot of work for myself, I decided to sculpt my own.
I am no sculpter! However this was pretty easy and may help someone in the future. I also did this with no tools other than a hobby knife. I made an assault pad which I'm going to cast which followed the same principle. So to start I spread some greenstuff over the pad and made sure that it was fairly even, smoothing it out with my finger initially and then the wet blade. I made it about the same thickness as the trim on the pad.
Once you have that done you do the same thing on the inside to remove the left over material. Again, put the tip of the knife where you want the cut to start, and hack down. Try to make sure the arms of your sculpt are the same width.
After you have this done, make two cuts where I've shown on the left. This will give you a nice hard angled sculpt. Now that it's done, you can go back and push the side of the blade into the edges to make sure it is square and has hard edges. After it hardens you can even trim it up a bit or smooth the surface further by scraping off any lumpy parts.
Again, I'm no sculptor but this was easy enough for a half decent result. I realize there are many little sculpting tips out there so hopefully this gives you somewhere to start or the confidence to try more difficult things.