Generally, those containers are for use in a "breeder tank" (or so I think). The slots in the bottom allow for the fry to excape from the mother...but they need to fall into a tank where they are safe, like a small 10 gallon or something. Many times people will make these small tanks with no gravel or decorations, simply because it makes them easier to clean with the fry. When I had my first few guppies and plays (which are fun too), I bought one of the breeder "nets" which is the same type of device but the fry do not excape, and you have to remove the mother when she is done. Eventually though, after the first round of fry grew up, I coudlnt even keep track of all the new females that were pregnant, and it was impossible to catch them all and put them into these containers (and I dont use one of those breeder "tanks").
If you want my strategy, I bought a clump of java moss (which grows in almost any condition) and left it in a ball and stuck it in the corner of the tank. It started out as a size of a golf ball, and is now the size of a softball. The fry stay in there and it provies a sorce of nutrition and shelter. To be honest, i started out with six in a 20 gallon, and this week im transferring them into a 55. I probably have close to 40 fish now (way overpopulated in a 20).
Like the others have said, you lose some doing it this way, but in reality, thats ok.
I also have a betta in the tank, as another suggestion was made. They eat the fry and also add color?
Hope that helps!