If she isn't interested in the box you choose as her nest box that is okay ^^ sometimes mom cats will ignore nesting places until they are in active labor then they will settle. If you can I'd suggest setting up a few places for her to nest. Any place she can hide or there is a closet are all places she might choose to have her baby so look for those places, and prepare. Put a blanket or towel down (something you don't mind getting dirty/bloody) unless you don't want her in those places, then block them off if you can.