best case scenario you can use to reshape the plastic isn't even boiling water...the best Idea is to fill your sink with cold water, and then on the other side have hot water running, then put the armor under it.
It doesn't take alot of heat to reform plastic. Basic plastic manipulation 101:
It takes only a small amount of heat for the particles in plastic to begin to move rapidly enough to make it pliable. IT will make it soft enough to bend but still have a rigidity to it as to not lose its initially formed shape. As you have noticed intense heat will actually change the physical property of plasic very quickly. You can literally turn plastic into a liquid very quickly. It can reach a semi solid state in a matter of seconds so you have to be careful with the range of heat you are using.
Now for the bending, you just keep your armor under the running water for a few seconds - a few minutes depending on the thickness of the armor. This will give it enough give to bend it in to submission, then when you have it, keep holding it with that bend and dunk it in the cold water, this will cause the particles to halt suddenly and hold its shape to the new form.
