The one small bathroom in my Spanish-style house had a cabinet with a seriously warped door. When closed, the bottom corner was about an inch out of plumb.
At the time, I was building banjos and had a supply of adjustable truss rods. I decided to use a pair of them joined by a threaded coupling as an external truss rod on the inside of my cabinet door. I found a piece of aluminum angle and cut a pair of anchors from it, drilling four holes in each so that I wouldn’t run the risk of them pulling out of the soft wood of the door frame.
As shown in the drawing, the two rods span the back side of the door, running diagonally from the bottom corner of the frame. As I tightened the rod, the tension drew the two opposing corners toward one another, relieving the twist in the door. The door fits perfectly and hasn’t required readjustment in 30 years.
By the way, you don’t need a banjo truss rod to make this technique work. Any threaded rod from your local hardware store will do the trick.
, Palo Alto, CA