It sounds great that you know exactly where the ham is! As Serendipidity said, if it's a vent system, you could place a bucket below the vent opening with a tube leading down to it and some food to attract her in. I do hope that you get her out soon
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Minion to Daisy, Lily, Harry and Peanut <3
Gone but never forgotten
Willow - 18:27 18th November 2016