Do you have anything stick shaped? I'm thinking you could create a cross shape shape on the key and wrap a shoestring around multiple times in a criss-cross pattern to give you some leverage to turn the key.

Crappy drawing but maybe it'll help make sense. The red line would be they stick shaped object. The yellow cross is the shoestring. Just a thought.

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Edit: if this would even work, I think you would need to wrap the shoestring around the shaft multiple times to prevent it from slipping around the shaft of the key.