Step 4: Foam Prototype
ALWAYS WEAR EYE/EAR PROTECTION WHEN NEEDED.
Secure your foam blank in place on your tooling. "Plastic is playing." In other words, use plastic bolts and washers unitl you know for certain you aren't going to hit them. Better safe than sorry.
I again have to assume you know how to run your CNC router. We used our schools old and worn AXYZs. Our foam prototype was 1/4" off on either end of a pass, from a slipping Z drive. The result of student abuse of our poor machine that led to its recent untimely demise.