There was a girl who made casts of the willies of famous people she slept with.

Just be very careful, as a girl who was making a plaster cast of her hand here a couple of years ago used the wrong plaster and the heat of crystallisation actually burned the skin, quite badly. Not the sort of thing a bloke wants to happen to his dick.

Once you have the cast, you can experiment with what makes the best substitute.

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Calm, quiet, smooth, devastating