Will This Diy Breeder Box Work?

i know there is a diy section but this also has to do with live bearers, i made this so the 3 pregnant guppies babies can go without getting eaten for a while. do you guys think it will work? i tried with molly fry but they never got out. but this was without any other fish in there, so they had no reason to leave. what do you guys think?
 

my one concern would be the amount of waterflow through it to allow proper air exchange and so it doesn't become a slighty-concentrated with ammonia and kill the fish. I say just drop the extra 10$ on a net breeder box

With any breeder box, even the manufactured ones, I flush with tank water daily/twice a day if I keep fry in them. They can never have enough flow through them in my opinion, even a net breeder. It gets covered in some biofilm and small particles from the water, and eventually provides no flow at all.
Go with what you have, cover the bottom with a thin layer of gravel/sand, water will still go through but the fry won't escape. Add some cuttings from live plants to provide cover and additional micro fauna the fry can nibble on. And flush tank water slowly each day, morning and evening(or whenever). The left over food can be removed with a syringe. That's why I prefer sand, because the uneaten food becomes like fluff on top of it, easy to see and suck up.

I have read that you can take those strawberry cartons with slits on all sides. hold them in the tank on the sides and as the babies are born they fall right through the slats and the parent cant get through them. Have no  idea if they actually work but it sounds to me like it could. Just a thought from a noob!