Couldn't find a decent photo of a pregnant guppy but found one of a molly. Th squaring off is the same in both anyway.
you can see at th back that she's kinda square rather than round.
One thing that i would advise however is that the breeder boxes are highly stressful for the mum and not good for the babies for more than a day or 2. Placing the mother in the breeding box could cause her to abort the lot.
If the tank is heavily planted then most of the fry should survive and my experience with guppies is that even in a bare tank many of th fry will survive nayway, at least half will avoid being eaten as long as the fish are fed.
Another thing i would mention is about what Lupin said. What will you do with all the babies? You will either need a huge tank or multiple 10g tanks to cater for all the babies if you don't want to risk an amonia spike which could lead to killing off many of the fish, babies and mothers.