Nylon fishing line(thin thin thin stuff) can work well too, just have some c-stand arms to hold/the ends. Position accordingly for best angle.
Otherwise, try using some rigid wire, something strong, like copper.
The big issue is that you want to 'sculpt' the shoes/sandals so they look like they're filled with a foot, not flat and floppy... Make sure t bid enough time in, see f you can borrow a difficult pair or two, and try it out. Time yourself, and then add another 30% to that.
Patience and air conditioning can make this an enjoyable job! One of my first assisting jobs was working with a studio shooter shooting still life's of sandals, many of which were like the one in your picture. With careful planning, a stylist to aid in making sure the shoes looked their best, and good lighting technique, the 3d affect was quite noticeable in the resulting shots.
We also used a 'lazy susan' rotating platform to rotate the shoes/sandals on axis, the shoe perfectly centered.