I Have A 4 Month Old Orphaned Kitten. She Will Use The Litter Box Only When She Wants, How Do I Get Her To Use It Constantly?

She will use it if she is in the room, but when she is upstairs, she will just go in whatever room she is in? I have tried to cage her for a few weeks, and she uses the litter, but then when released, will go where ever she is, again. HELP!!! We don't want to get rid of her, but can't have a litter box for a home.