Most of the door bookcase construction is the same as the stationary ones. The big differences are 1) the door wont be resting directly on the floor and 2) it will be moving so i put an additional piece of 3/4 inch plywood on the back, bottom and top. For the bottom, attach a floor fitting to the corner that will pivot. Attach a string or wire to it and use as a compass to mark the curves that the casters will lay on. I attached the casters and fitting to the second piece of plywood with bolts and attached it to the other piece plywood, hiding the evidence. attach the pipe and another floor fitting and move the bookcase into place. Once in place, unscrew the pipe until the second fitting reaches the floor. Mark the holes position on the floor. Move the bookcase out of the way. Drill at the marked points and insert concrete anchors. Put the bookcase back in place and use lag bolts to anchor it to the floor. It will now hinge by screwing or unscrewing in the threads. Since it's only a quarter turn, there isn't a significant change in height. I repeated the pipe flange setup at the top to keep the case from wobbling. Just take careful measurements to put it in the same spot, or it will bind.