Several comments have asked how my TARDIS model was put together. So, eight months later, here's a video showing the structure of the model. I hope it explains things.

The model has three mirrors two cornertocorner diagonals, and one larger mirror at the back of the box.

The diagonal mirrors reflect the side walls, making it look like there's space behind the TARDIS. They also have the outline of the TARDIS body cut from them, so the interior can be seen through the back of the TARDIS.

Only half of the interior has been built, so the reflection in the back mirror doubles its size and creates the illusion of the box being bigger on the inside.

The model's had a few knocks since the original video bits and pieces are falling off, and the mirrors are starting to warp and my shoddy craftsmanship is more clearly exposed! :D