With a little money, resources, and time one could create the initial concept for a working TARDIS. I was looking at the PORTAL game series and it got me thinking about the connection of the portals themselves and I think now that the TARDIS could be made as a structure before delving into creating it as a craft.
The process would need to start out with the creation of a remote facility that contained the main power supply to run the facility, its living quarters, and production areas as well as power for a temporal gateway at a specified entrance.
If you look at the TARDIS as it is portrayed, the initial police box is completely there and the contents of the control room doesn’t sit within it but rather at the back wall and beyond. So, therefore, by creating a doorway with one frame as part of the facility and another frame as the back of a police box you get the initial illusion that the police box has a limitless space within it.
Now, the doorway would be the portal generator and the facility-side would receive it’s power directly through the control room from the main power source. The two frames would be connected physically to each other as to transmit power from the facility-side to the police box-side to maintain the portal and to allow for future features of the craft.
When energized, the frames remain connected but now the police box-side can be moved to another location as the portal continues to provide a constant link to the facility and its power source. By starting out this way it would eliminate the need to make dimensional calculations of the facility entryway origin point and give critical testing information on how to track the two points as they move past each other in space.
The only preventative measure at this stage would be to keep one from trying to pass the police box into the facility and through the portal from that side. The portal would surely create some interesting but destructive event.