Perhaps the most tricky part of creating Leia's bounty hunter costume. It's best to have a friend to help you with this...
The goal here is to create the shoulder/chest piece out of Wonderflex.
While you can opt for ABS plastic, or plaster of paris, or some other type of material -- I recommend Wonderflex due to the adaptability. You can literally tweak it through reheating to get a perfect fit.
For info on finding Wonderflex, please look at my helmet tutorial.
I recommend making a mock-up first out of poster board or cardstock. It won't provide full curve-hugging abilities, but it will give you an idea of length, width, and height of collar.
Check out reference images to get a good idea of what you want your armor to look like.
Also consider factors like collar height, bust size, and the thickness of your costume's soft (fabric) components.
Shoulder to mid-bust (or length of armor you desire)
Shoulder to mid-back
Collar height, based on neck measure in front and back
Mid-breast to mid-breast
Shoulder to Shoulder
Draw a sketch of front, side, back with the measurements written in.
Cut 2 pieces for the front sides based on measurements from mid-bust to shoulder (length) x shoulder edge to neck edge (width)
These pieces = the front part of your armor.
Cut 1 large piece for the back part of your armor.
mid-back to shoulder (length) x shoulder width (width)
Cut your collar piece.
neck circumference x tallest height in the back of the collar
Subtract about 2 inches from the front and angle the sides upwards towards the rear.
Cut small inlets at the bottom of the collar where it attaches to the rest of the armor. Flare these out to create an easier surface to attach.
To create an extra-rigid piece of armor, you may want to double the layers.
Using your heat gun and fingers, turn all external edges inward, creating a rounded edge.
Attach all pieces together.
Keep the old t-shirts on to keep yourself from getting burned and guesstimate costume thickness, and have a friend gently mold the wonderflex to your shoulder area.
If you don't have a friend, use a stable door and lean into it. Not the best idea...but it works!
This is what you should end up with.
Here is an interior view of the bonded armor.
Note: yours may not be totally symmetrical. Mine isn't because apparently my body isn't. Don't freak -- if you opt to do the single-layer armor, it will adjust to fit you surprisingly well.
Like the helmet, it's time to cover up the wonderflex's duct-tape texture with the air-dry clay.
Sand, apply more clay as needed, sand, and then paint. I painted inside and out.
Optional: The cannisters on the back are upcycled tin cans and heavy-plastic drinking cups sanded, painted, and riveted together.