Lets start with the drilling, a word of caution; if you set the dremel at a high speed YOU WILL MELT THE PLASTIC, start with the lowest speed, first do a straight hole (1). Move it a little so you can expand it but be careful the purpose is to take out as much plastic as you can not to hollow it completely.
Now with the small/slim grinding stone you can start sanding it down from the inside (2), remember SLOW SPEED!!!, the bigges secret to NOT damage the armor at this point is TACTILE SENSITIVITY, the stone friction is making the plastic rise it's temperature right?, that's your signal to know that you are using to much speed; also it helps you know when you are about to breach the exterior of the armor.
How much are you supposed to hollow the armor?, use the arm to know how much sanding is needed, The piece should look like this (3) and this (4).