Is it worth possibly relocating the mother in advance to the future fry tank? That way she can drop in there and you can move her out instead? You may have ruled that idea out already judging by your first post, and helterskelter is certainly the one to know whether it's easy to remove the fry once she's done.
I just know that if I had a choice I'd probably choose to have the female in a birthing tank in advance rather than try to catch fry after.