I think physiotherapy is important so I would not use a sling all the time, otherwise she will just lose muscle strength. Placing her on a soft surface like grass and encouraging her to walk towards food or some other treat on a daily basis and extending the distance each day would be good to try to maintain her mobility, but if it gets worse, then you could perhaps experiment with cutting the bottom out of the box and fitting it with wheels so that she can propel herself around on a level surface. You will need to watch that her knuckles don't develop sores if she is walking with her toes curled under.