Can you go into further depth about what you mean? If you bevel the foam(cut at an angle) you can just add a flat peice and it will create the geometry shown in the center of the chest by the neck that looks like a Y.
Most of the detail on the armor are peices raised or lowered into the foam do it is on a different level. For the edges, you can either glue a seperate peice sideways against the foam to give the almost 90 degree corner, or bevel two 45 degrees so you can glue them together to get that angle. I hoped I helped, and I dont even know if this makes sence...