I have a "Skirt Removal Tool", made from a brass bar and some styrene strips. The idea is, take a piece of 1/4 x 1/32 inch brass bar, about six inches long. Take some heavy styrene strip, I used 1/4 X 1/16 inch, and make two angles from it, each about 1/4 inch long. Glue the angles to the bar, about where the body bolsters are.
____________
|________| |
|________| |
| |
|_|
The vertical portion keeps the bar against the underside of the truck cut-out, while you cut the skirt off, and everything stays straight. I actually use the back of the blade, as it removes the plastic, rather than just pushing it aside.