I would use the fry tank to let her have the fry in. Just put some fake plants in it, bunched tightly in the corner. The fry will hide out there. Even float some for extra protection. In my experience, it's nearly impossible to save ALL the fry but you will probably only lose a few this way. They usually, but not always, give birth late at night. If you check the fry tank every morning, you will see when she has had them, so you can then remove her. She won't be able to eat too many of them by the time you notice. She will be considerably smaller as soon as she has them....
This is the signs I look for when she is ready to be moved to the fry tank.
-She becomes very lethargic
-She stops eating.
-Starts hanging around the top/bottom of the tank.
-Stays close to the heater.
-Her anal area will start to swell/bulge and will open slightly(dilate).
Make sure you cover the filter intake pipe with a prefilter sponge or some NEW cheap pantyhose. It stops the fry getting sucked into the filter. When you move her into the fry tank, try and keep the lights off. This will keep stress to a minimum. If she feels stressed, she can hold off the birth or may even abort. I keep my fry tank at 80c, this helps the fry grow faster....