I was intrigued by Angelee's observation a while ago that her guppies were dropping at night - and I believe it was she, like you, who was checking at some pretty late moments. (Always minding the kids...) Guppygirl notes that her guppies drop often in the morning.

I wonder if there is an advantage to the guppy fry to be born in the gloaming before all of the other potential predators in nature or in a tank are active.

At any rate, feed your female(s) well on some meaty foods. Defrosted, rinsed, glassworms or bloodworms or a few live blackworms left in a clean (soapless) pickle jar could serve to fill her up, provide her with enough nutrients that to keep her from eating newborn fry. That is why the worm in a jar suggestion - she will almost always go for them before the fry. The worms are right there - and they can't climb out the glass.

"Waddling" guppies and platys don't hunt fry much. ;)

I had some livebearers drop the other day. In addition to their regular foods, they have gotten a couple of helpings of blackworms and even a couple small squirts of daphnia - which will swim around the tank, cleaning the water of algae and bacteria and even reproducing until eaten by the fish - hopefully in lieu of the fry.

If the other guppies and the platys in the tank are also well fed, they will be less likely to forage for fry.

What you have there will be useful in providing shelter. Roughly speaking, the more shelter you have, the more fry you will find will survive.

If you can locate it, Java moss is a terrific bottom plant and would be an excellent place for your fry to hide out and search for microscopic foods. (Even those clean, soapless plastic pot scubbers, unwound, can offer some shelter.)

However, some fry head for the surface. (They are somewhat phototropic and head for light.) Can you get some water sprite from another aquarist or some hornwort from a shop? Out of sight, out of mind is a good rule for somewhat hungry livebearers.

You considered isolating one or both of your female guppies. That will work, even without a lot of hiding places for the fry if she/they is/are really really full of alternative foods.

By the way if the potential guppy birthing is driving you nuts, just wait until the platy females start dropping two or three fry a day for a week or more. Their pattern is often much more spread out than that of the guppies! :0