Do you want it to function like a mousetrap car or only look like one? The main difference is that mousetrap cars use the trap to create the propulsion. Without it, the car doesn't move. If you were to build something out of lego, knex or even Meccano (also known as erector sets), you'd probably have to glue, or otherwise secure the parts together. Perhaps you could just add the spring mechanism from a mouse trap and secure it to your other parts? Here is an instructable for a Lego Mousetrap Car. They've used a mousetrap in the design. Here's another site with links to resources about the construction and design of a mousetrap car, if you need more information.