In the box!!! Duh!!!
Lets not forget the effect of Metamorphisis here people!!
The effect is that the magician has been captured and securely trapped inside a box and often in a lot of other stuff too. The captor stands triumphantly over the box holding the magician, and then shazam!!! The magician's magic frees them and traps their former captor inside the box trapped away powerless against the magician's magic.
Its not an escape, Metamorphisis is a magic effect, and magicians that start on top of the box and then go inside it are screwing up the effect.
A magician going inside the box in Metamorphisis is no different than doing a color change with a spectator's card backwards. Is this your card? Specator says yes. Then the magician changes it to some random card. "Now its not!!!" Its much better to change a random card to the selected card because if you do it the other way, its still magical, but you look kind of stupid, dumb, or strange.
That actually made a lot of sense. In my opinion you should start in the box too.