Guppy Fry Container

Hello. I'm a teenager who recently started a aquarium in my room. However, my family has had fish for probably 10 years, so I do know some things about fish keeping. I pay for all my fish, so I have a old tank from 2019, but I do try my best to work hard to afford things for my aquatic friends. My tank is 29 Gallons, and I do have a filter and heater. I own two loaches, 3 ghost shrimp, medium sized community fish and 2 guppies. I got the guppies very recently but they seem to be doing well. All the fish I own a male tuxedo koi guppy (hence my username) named Koi and a male turquoise guppy named Peacock. I did my research and realized they do best in groups of three, so I am working on getting more very soon. With new guppies, I was intrigued by getting females with them to raise fry. I've done my fair share of research, but havent gotten a clear answer. How big does a container need to keep a single batch of fry? Im asking now so I can prepare for the future! I can't quite afford another tank, but we have plastic containers in our house. Is it possible to use that? I also heard of breeding boxes but I figured it would be easier to have another container.
I appreciate all responses.