Hollowing Out Parts...

I'd start in the middle and work my way out towards the edge, that way you have more control and don't grind something you don't want (figure or your hand)

I do something similar but instead of using the grinder stones I use this metal bit with like a spherical head with this grooves cut in it, and I essentially just bore-scoop out the plastic and then smooth it out with grinder-polisher bits.

I Did This W/ My Marcus Fenix Body Armor.

You Need a Dremel , Safety Glasses , Cone Grinder Bit-red stone , Needle Nose Pliers, Hobby Knife , Patience.
1. Safety Glasses ! - Rip Figure In 2 Or What U Wanna Hollow Out.
2. Start Dremeling Slowly . ( Going To Fast Can Burn Through To Surface U Don't Want That!)
4. The plastic Will Start To Melt And Smoke
(Hot Plastic Will Adhere To The Bit so Remove bit from plastic & Use Needle Nose Pliers To Remove the Cooling Plastic From Bit & Use Hobby Knife to cut Off Excess Plastic From Bit .)
5. Repete Till your Satisfied .

I Hope this Helps !
Work Safe
&
Good Luck !