TARDIS Exterior: The Curtain(s) TARDIS
Why I went with this design: I imagined a TARDIS that would hide behind a classic red curtain, which you can access by pulling it aside. I used the traditional red fabric theme because I think that's what everyone thinks about when one says curtain. I submitted two designs for this entry: one with only one curtain, and the other with two. I couldn't decide which version was the best so I showed both.
Images:

Honestly, I have no idea if this exterior would be better as a 3D model or as a flat texture on the wall (like the current Portal exterior), so I guess it's up to you.