No doubt there is some sort of dowel support. Just insert a thin dowel or wooden skewer in your slipper before it dries. Just make sure your skewer is long enough to hold the weight of the ballet slipper without it leaning. Placing the 2nd slipper behind like in the picture could help with that too.
Thanks for the tip about making sure the dowel or skewer is long enough,makes sense.I will just give it a try.
As a general rule, you need something strong enough to hold what you're trying to stand up. Something big needs a dowel, but for more dainty things, then spaghetti works well.