When we found Bramble we placed her in a box (Well a chest of drawers on it side, with all the drawers removed.)

We lined the carpet with several layers of newspaper. I found a very shallow plastic plate (easy to climb into) with a small amount of litter and Bramble used it straight away. I think cats have a built in, litter/dirt is good to go on potty on !!!

Maybe you could try a shallow plastic plate, as a litter tray for such a young cat.