Use this kit with a couple of old Linux distros, a drinking straw (or other axle), and some monofilament (fishing line) to build a mousetrap-powered car. There are 17 parameters you can tweak, but you'll probably be most interested in trapThickness and axleDiameter.
The STL is rendered for a trap that's about 6.1mm thick, with an axle diameter of about 6.25mm. Change the trapThickness and axleDiameter parameters in the OpenSCAD file if you need different sizes.
Use hot glue to fasten the wheel hubs and the pulley to the axle. Thread the fishing line from the top of the trap under the axle and then wrap it around until it's tight and the trap is set. You may want to use a bit of tape to get it started.
WARNING: Don't use rat traps -- they can break bones.