The police box Tardis is the stuff of legend of course, including its origins being due to low budget and the like, but suppose the series was based in modern day America to the same extent the current series is based in modern day London. (Yes, the whole point of the series is the Doctor can travel to the age of pterodactyls to 5 Billion A.D. and to pretty much anywhere in the universe, but his companion is usually from/he always returns to the 21st century U.K…)
Since police boxes have never been part of the American urban landscape, and Bill & Ted have a claim on the time traveling phonebooth, was would be a good disguise a U.S. based Tardis? It needs to be something that would be relatively unnoticeable in most landscapes (with all the “timey wimey wibbly wobbly” disguises itself if visiting 3rd century Vietnam or the Lobster Men of Titan setting in place).
I’d proposed a tin or plyboard shed. In a rural landing it would look natural- a tool shed or hunting shack or whatnot, and in an urban one it could look like part of a construction site. I like tin or scrapwood because it could look as natural in 1835 as in 2012, but for fancier there’s always the prefab types they sell a Lowes or Costco (example).
An airstream camper would take up more space, but I like the aesthetic. Friends I asked suggested a VW bug- possibilities there- or a dumpster. I don’t like the idea of a portapotty because too many people would probably try to use it.
I’d go with a car. It fits in anywhere. What type of car? It seems like silver/gray compacts are the most common around here, with most models having the same basic shape. Maybe something like a Kia Spectra that looked around 5 years old.
Dumpsters are everywhere, but not empty ones. Nature abhors an empty dumpster.
I wouldn’t go with a car. It’d get ticketed all the time or towed away, or people would wonder why it wasn’t having that happen to it. And you can’t run headlong into the open door with someone holding it open and screaming ‘come on, come on!’ Also, they usually have windows that you can see into; lots of countries have laws about the front windscreen at least being clear.
The police box was supposed to not stand out, but that always seemed like a strange reason to me - a British police box would stand out almost anywhere except for a century two in Britain. I suppose that doesn’t matter too much, since so many episodes take place in that time and region.
But it’s also not the kind of object people would expect to be moved along. So a portaloo would be a good one - just have an’out of order’ sign always on the door. Then you could have an episode where the Tardis’s magic isn’t working or the Doctor’s gravely ill or something, and a subtle sign of how serious it is is that the sign disappears.
There would be confused comments about how that Doctor seems to have some sort of bowel problem, since he’s always in the loo, and it’d look very dodgy seeing him go in there with his companions. Plus the companions could occasionally run headlong into the wrong portaloo.
A refrigerator box would probably work, but I think the perfect Tardis wold be a single worn-out sneaker, men’s size 9-1/2. I just saw one sitting on the shoulder of my mostly deserted county road next to a culvert and didn’t think much about it except, “Heh, single shoe.”
If it has to be bigger, then one of those green metal telecommunications switch boxes you see all over, city, suburb, and county. You know, the ones that are usually covered with graffiti. (I wanted to link to a picture, but I can’t make Google understand what I’m looking for.)