I agree, breeding traps are just fine. When my females look like they are going to drop, especially if I find a fry in the tank or the female is hiding a lot, I put her in the breeding trap. If she doesn't have any (more)fry in a day, I release her. Then put her back in once I see more fry and leave her until she done. I haven't lost any mommies yet.
I hope you have better luck with your platy than I do mine. I have a dwarf platy that is that huge and it's been a month that she's been taht size and STILL no fry!!!! Good luck and congrats!