I personally have the mesh net breeders (3 dif sizes), floating acrylic boxes (4 sizes), a Ziss BL-2, Ziss BL-3 (which is the one the Co-Op sells) and the hang-on breeder boxes in medium and large - though I only use the large size for fry , the medium was turned into a bio-filter.
Mollies (along with other livebearers such as guppies, platys, swordtails etc), reproduce through a process called Superfetation. (Superfetation is the simultaneous occurrence of more than one stage of developing offspring in the same animal)
One mating, can lead to the succession of 6+ different broods / batches of fry.
The females will give birth every 28 to 34ish days (depending on temperature, diet and other various things)
So don't feel bad if you decide to just let them be in the main tank! There will be more to come!
My personal recommendation, is firstly to not put pregnant / birthing mamas in the box/net --- but instead use the box/net to hold newborn fry you find!
As far as the type of box goes - the hang-on boxes are definitely nice for when doing maintenance and not needing to worry about water level and whatnot. but I strongly recommend adding an ainstone into the box (place in the center)!!!
^^^ also don't recommend getting anything smaller than the large size
The Ziss boxes are definitely neat. I personally like the BL-2 version better though. It allows me to add a thin layer of substrate, and so it allows me to be able to place/plant a few small stem plants in the box with the fry. The built in air stone is also nice.
Whatever type you ultimately end up with, provide cover for the fry that are kept inside. This can be done using live plants (stem plants, floating plants, and of moss), or, you can pop off the "leaves" of the plastic / fake plants and those will float, too.
