Having hand raised a couple of kittens that just had their eyes open, I took a small box and cut the sides down and placed it and the kittens in a larger box at night with an area for sleep and at the far end the "litter box". I placed a drop of ammonia in it as I have always done to train kittens or cats. They both adapted to that and after growing some to where they could get in a regular litter box I never had a problem. Worked for us, hope you find something that works for you.