There were all sorts of headaches trying to marry up the cube and the pose. When I shift Esther a centimetre one way to make one part of her interact properly with the cube, immediately another part of her interacts wrongly.
Also, for the optical illusion to work, the cube needs to be near symmetrical, which meant perspective was redundant, and so I couldn't shift any vertices either
In the end though, of all the poses or images I could have tried to marry up with the cube, I suspect this was one of the more workable ones and that if anything, I actually got quite lucky.