Hi, I have bred guppies on a large scale. I personally like the females to have their own tank. If you have a 1 or 2 gal tank that could be set up with aged water from an existing tank do so quickly. No need for gravel, filterand heater a must you can use those round sponge filters, I just loved them as they did not trap and kill the fry when they were born. Put in a few plastic or live plants for the fry to hide in.
I would put the female in there when they looked like your is. I would also keep the light off so the female could not see the fry is she had them during the night. They will eat a certain amount of their own fry if they can see them. In the morning and during the day I would watch to see if she was having them or not.
After the female had dropped her fry I would remove her to a seperate tank for a day to get back her strength and then into the main tank again she would go. A single female can store enough sperm for approx 5 batches of fry without ever being in contact with a male.
The fry would stay in the small tank (1 or 2 gal) and they would be raised in there till of such a size. I would then cull, seperate and put males in one tank and females in another tank for grow out.
I am totally agains the breeding nets that hang in the tanks as they stress out the females to no end and I had alot of dead females this way. Your seperate tank is your best bet if you can get one.
28 days is usually the amount of time it takes before they drop fry. However if they are stressed they can drop early ending in alot of dead fry and they can also hold without dropping for several months. Had both happen to me when I first started.
Sorry to say that the container you made the fry will just fall through the holes and the bettas will be right there for a tasty snack.
Do try and get one of the tanks mentioned above if you want to keep some fry.
Good Luck....bugsy