If you don't have a band saw, use this 3D printed guide with a bow saw to cut your Pinewood Derby car to a cool ~10mm thick.

Four screw holes on the outer perimeter (1/4" or 7mm) are used to secure the jig to a piece of plywood (or similar). The pinewood derby car slips inside, axle slots should be on the same side as the bow saw cutting guide (resulting in a ~10mm thick car). After cutting, unscrew the jig, flip it over and use a screwdriver to push through the 2 holes on the bottom (back & forth) to wiggle the car out of the jig.