Along with what these fine gentleman are suggesting for drill bits, Here is what I did. After drilling out the majority of the plastic by hand, I did not make the plastic thinner but left it as it was, I used diamond coated finger nail files that I found at one of those cheap places like the dollar store (2 for $1.00). They are very thin, one side is course and one side is fine, I used my dremel to shape them to the width I needed to fit a particular opening. Now you don't want to get the file too hot when shaping it as the bonding material will flake off and take the diamonds with it.
Hope this helps.
Put your heart into it, well done is better than well said... Ben Franklin