In reply to FraserCam: as people mentioned above, you can hang fingerboards off one of those expanding chin up bars that dont need any fixings. However much simpler way of doing it is:
1. mount the fingerboard on a bit of pretty stiff wood that wont flex, that is a few inches wider than the door frame.
2. drill 2 holes at the top, a bit narrower than the doorframe width. Thread some 6mm cord through and tie a big knot to hold it.
3. make a couple of grooves in the bottom of the wood board about the thickness of the cord, in line with the top holes.
4. tie another knot, so the cords (one on each side) basically goes from the hole in the top, over the pull up bar, and hooks onto the groove at the bottom. The wood baoard being wider than the doorframe means it doesnt swing, and the cord attachment means it's really easy to take on/off when needed.
Does that make sense? I'll post a pic of mine if you want