I just feel bad if it’s due to stress or inadequate boxes
Oh, quit that. It's not her stress or inadequate boxes because your others are using them. No need to feel guilty. Sometimes chickens do what they want to do, not what we want them to do. That doesn't mean you did anything wrong, just shows that with living animals about anything can happen. It's happened to me before, more than once. I don't take it personal, it's just something that sometimes happens.
If you are OK with her laying there, you are golden. No need to worry about anything.
When I have one laying where I don't want them to I wait until I can catch her on the nest laying an egg. I catch her and lock her into a real nest until she lays an egg. I made a couple of my nests so I can lock a chicken in there if I want to. That's come in handy for a few things. It generally takes them about a half hour to lay the egg, then I let her out. Usually doing this once is enough but I had one stubborn hen that I had to do this twice before she got the message. And it took her three hours to lay the egg each time. Did I say she was a stubborn hen?
I haven't tried this but some people say it has worked. Make a nest, maybe out of a milk crate or a box, and set that right over the nest where she is laying. If she keeps laying in it, after a few days move it a foot closer. Wait a couple of days then move it a bit more. Then either move it to somewhere you are OK with her laying in and leave it there or when you get to the real nests, remove it and see where she starts laying. Hopefully it will be your real nests but I can't give guarantees.