Fully functional, and furry!
Warning: this project involves taxidermy, dremels, and sometimes graphic pictures of dead animals. While there are no guts in this tutorial, viewer discretion is still advised. If you are offended by this entire idea, please peruse the wallet contest or the laser cutter contest instead.
Concept by noahw
Implemented by noahw and canida.
Step 1: Acquire mice
Obtain a similarly-sized (wetware) mouse. These are commonly available fresh or frozen from pet stores, or any other place that sells reptile food. It's easier to fit a small object into a large mouse than a large object into a small mouse, so err on the side of caution. You can always fill extra space with cotton balls.
If you've got an optical (hardware) mouse, make sure to choose a pale-furred (wetware) mouse for lightest skin pigment - this will be important later.