Ah yes - the hamster dictates the toilet area! They will only use a litter tray if you put it in the place they have chosen
Interestingly I found if you give them a large house, they move their toilet corner inside the house - as such I always have a large house now with a corner litter tray inside it
If the house has a lift-off roof you just take the roof off and empty the potty every five days or so and the rest of the cage stays clean and dry - apart from the odd wee in the wheel of course.
Anyway giving them a large house isn't compulsory. If the sand box is too big to move to her chosen corner then maybe add a corner litter tray as well in that corner.
I've found these good - not too big or too small. They tend to pee in the back corner and sit in the front for a wash
https://www.amazon.co.uk/Trixie-Corn...=hamster+potty