This is an intriguing question, especially because it can have multiple answers!  What comes to my mind when I think about it is that Corrie is like her father's clocks - especially his precious astronomical clock, because of her steadfast faith.  Over the years, the clock has stood, "tall and gleaming on its concrete block...shorn...of eminence" (Chapter 1).  Under father's loving care, it has ticked on faithfully, never missing a beat, keeping time exactly.  The astronomical clock is a constant, something that endures and can be counted on. 

The astronomical clock can be seen as a symbol for Corrie, a humble, simple, completely ordinary woman who is deeply faithful to her God, and stands firm for all who come her way seeking help, and shelter, and stability.  Like the clock, she is dependable and steadfast, carrying on faithfully no matter what the vagaries of life may bring.

Casper Ten Boom takes great care of his astronomical clock, traveling to Amsterdam weekly to check the time at the Naval Observatory to make sure his own clock is absolutely correct.  In the same way, Corrie is vigilant in nurturing her faith, continually evaluating her own actions and praying that the motives behind them are pure.  As Father checks constantly to make sure his astronomical clock is never in error, so Corrie is unafraid to hold herself accountable to the clear standard of God by looking for outside help.   She seeks the advice and follows the examples of faithful others consistently throughout her life, especially that of her beloved Father and her sister Betsie.