I'd use a rubber cement. It's easy to remove when you want it off and won't mar the surface underneath.
But what's wrong with having a few boxcars running around your layout with the doors open? The big railroads didn't pay too much attention to open doors on empty cars. I mean, you gotta give the hobos a place to ride, too! Even loaded boxcars occasionally were seen with their doors open and the merchandise inside was visible from the ground.
I remember one boxcar with it's door stuck open, loaded with large household appliances, sitting on top of a bridge over a major highway and refrigerators and stoves were falling out and on to the road below. Now that would make a great operating accessory to one of our manufacturers to offer!
Paul Fischer