Protecting Swordtail Fry

[testingtesting123]

I have a female swordtail that gives birth to 20-30 fry every 6 weeks, but I have had difficulty keeping many of them from getting eaten as mummy fish seems to be hungry throughout birthing and will ignore flake and daphnia to eat her own newborns.
I've tried a net (about 10x6x6 inches) but had difficulty judging when she was finished dropping, and lost at least half of them. The best success I've had is saving 12 with a dreaded plastic trap with a lower compartment, but I still noticed a lot of the fry coming back to the top compartment and getting eaten. I've looked for a trap that would somehow only allow them out but not back in, or one with a way for them to escape into the main tank which I could section off, but I'm stuck for ideas.
On a slightly separate note, those 12 are 4 months old now and still all appear female. Shouldn't some of them have developed gonopodiums by now?

 

[OldMan47]

I think the worst of the bunch when it comes to not showing their sex properly are swordtails. I bought 6 juveniles at a fish club auction and all looked like females for at least 6 months after. By the time they had one start showing male, I had almost given up completely on getting a male. The fish were their full adult size and had been for months by then. Now they are close to a year old and I think I see a second one starting to look male.
I don't know whether they are made any more but I have an old trap that has 4 solid sides and the entire bottom is just plastic rods spaced closely together. There is a solid bottom that can be put inside that and a V shaped piece that can be used if you don't want the fry to go into the main tank. It is quite large and has a center divider in case you had 2 small livebearers that needed to be trapped at the same time. I no longer use traps because I find that I get plenty of fry by just giving the tank some dense plant cover.