You can score the plastic a couple of times then bend it and snap off. Go to a hobby shop that carrys R/C cars and ask if they have lexan body scissors. They'll have a short curved blade on them, and are great for tight corners and curves.
I found the scoring with a heavy duty exacto works best. I bought a set with a heavy duty and a light blade.
The dremel I used on the left over parts and once I brought that down, I used the sand paper to have more control.