This is an exactly proportional copy of the companion cube from portal, as traced from a screenshot.
There were a few cubes already on thingiverse which I've printed out dozens of, but they all had some issues with printability or proportionality. I think I've solved all the problems with this version.
This version is flat bottomed, and has no significant overhangs over 45 degrees.
It is carefully designed in Solidworks so any changes can be quickly and easily made. I also included step files since Solidworks is pretty unfriendly to a lot of other programs.
The default size is exactly 1in^3
The Cube in the thumbnail was printed by a Fablicator.
Print your companion cube, but be advised: area and state regulations do not allow the Companion Cube to remain here, alone and companionless.