The Snowtrooper Chest armor was custom sculpted by the artists at ILM for "The Empire Strikes Back". The armor was not cobbled together from existing items like some of the other costumes in the movies, it was all hand crafted from the ground up. The image to the side shows measurements taken from both a fanmade version of chest armor and from the "Magic of the Myth - laser caliper images" that can be seen in the Reference Pictures section of this web site.
Just so you know, there is no back plate armor on the snowtrooper. The backpack of the costume rests on the cloth of the trench coat and not on a plastic back plate. Earlier propmakers made back plate armor for the snowtrooper, but they were wrong in doing so. Check images the reference section and you'll see what we are talking about. If you are buying armor from a propmaker, deduct 10% from the cost of the armor for having a backkplate.
The chest armor consists of:
Discernable details to watch for when sculpting
Reference images donated by Jay Harris (TK-1010)
The rivet issue is pretty important due to rivets are generally not present on any Imperial armor in the Star Wars movies. The fact that the snowtrooper has rivets showing so prominently is a key detail in making the costume look authentic.
The chest is curved on the sides rather than being flat panels. The sides contour to the shape of the human chest. Amazingly though, the chest armor does not lay flat against the human form either, it actually juts out a good 8 cm away from the human body.
Study the images from the Reference Pictures section of this website before you start sculpting your chest molds. Take your time and you can have as good looking of a costume as ILM made.