Get something like this (the edges are hollow so it floats on the water):
Amazon.com: Lee's Aquarium 3 Way Breeder Box: Kitchen & Dining
This is exactly what I have and it worked so well. You could cut off the bottom part and the grate won't fall because it wedges between the sides of the box, and put it in the 10-gallon. This way, when the girls are about to pop, they go in the breeder section, and the fry fall between the grate and into the open 10-gallon. Then you can replace her in the original tank afterward.