I know you are pretty far along in shaping from the sounds of things, but here is some advice based on how I went about it if the heat gun continues to give you shaping trouble.
In my opinion a heat gun is great for some pieces of armor. Particularly when you are trying to bend a localized area or one side of the plate. To get uniform shaping for most of the pieces I recommend a different method. If you have a large pan...I used a 32 gallon stock pot...then fill it with water and set it till boiling. Grab a cooler of some sort, pot, or even the sink and fill it with ice cold water and a lot of ice. With this, solid gloves on both hands, and a pair of tongs you can easily shape material with little hassle.
After sticking the piece into the boiling water you can tell it's ready to be shaped because the plate starts to wave a little in the current caused by the boiling water. If I recall correctly it took about ~30-45 seconds for 6mm sintra and ~15 seconds for 3mm. The reason this works is the heat causes the molecules in the sintra to loosen up but not enough that the pieces are at risk for becoming waterlogged.
Once you have the desired shape on your body stick it in the ice bath and the molecules that make up the material will rapidly solidify so you don't have to hold it in shape for it to cool or risk warping because your hand slipped or got tired.
It's not too different a process from forging honestly! The only downside to this process is it doesn't work for smaller pieces and detail work. For this and minor shaping changes to improve fit...you rely on the heat gun.
