I use Orcaslicer.
I Orcaslicer (and I believe PrusaSlicer as well - or really any fork of Slic3r) allows you to import an existing STL to use as a "Negative Volume" on another object. Negative Volumes subtract, or cut, the geometry of that STL from an existing body. It's a lot like creating a mold or cast, but it's all done in the slicer.
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I positioned the bicep and shoulder armor the way I want and exported them together as a new STL file. Then I generated a cube in Orcaslicer and scaled it to the dimensions I want. I then imported the bicep/shoulder STL as a negative volume and positioned them over the cube.
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This can be done in a dedicated 3D modeling software. I simply did it this way because it's relatively quick. However, this method has drawbacks, mainly that while you can see the negative volume you don't see the actual finished product until after slicing. The feature itself is also slightly buggy (from my experience) - when importing an existing STL I've found that it often starts out skewed in size and way off the print plate so I have to move it and scale it to its actual dimensions. Trying to make a negative volume from two different objects and get them positioned properly can be tricky this way.
Thank you. That's kind of where I'm at - I'm going to try to get as much of it done as possible, but even if it's not perfect I know we can still turn heads trick-or-treating. 
