What Teajr66 said. I like the exacto for smaller spots and curves. Lots of patience! for me the cut out is the least favorite part of the build, and while I will get a speed kit on occasion, I can't usually justify the expense of a kit when you can cut it out yourself.
sharp blade and ALWAYS cut at as severe an angle as possible. multiple passes may be best, too. also, be sure to hold the blade at 90* to the foam, else the cut edge will be at an unintended non-square angle!