A CustomFixture is simply a fixture that uses the same custom message for taking, moving, and putting. In many cases, it's useful to customize the message for a fixture, using the same custom message for all sorts of moving. Just override cannotTakeMsg, and the other messages will copy it.
We haven't yet reached the point in our game where we need a CustomFixture, but we'll eventually use one to represent the pillars in a temple.
See also the similar but subtly different CustomImmovable.