Have you ever wondered how some modelers make such "PERFECT" wheel wells? Well, I learned a little secret that can help you achieve success every time. The OLFA Compass Cutter model CMP-1, is the key:
It is an inexpensive tool(less than $10 bucks). It adjust to a diameter of 6 inches. It is really easy to use. Simply mark out the center point of where you want your wheels to be. Lightly press the point or drill a small hole where the compass point will go. No worries, as this area will be removed once the cuts are made. Place a small piece of masking tape over the hole. This will keep the compass from "walking". Lightly rotate the tool around the diameter of your styrene sheet. With each pass, go a little deeper. Soon, your plastic disc will be free. You can use this technique on single wheel wells:
You can order the tool directly from OLFA, or purchase it locally from craft or hobby shops. Replacement blades are also available. Hope you find this useful.