If you have a bigger building for them, you can suspend the brooder shields in the building, taking care to provide a draft shield during the first several days. Hope this helps. Let us know how you do. And I should add, regarding the animals. . . They need to be kept out of the area where the brooding is going on.
Chicks move, and that movement is fascinating to cats and dogs alike.
A box three feet square would only serve a very small number of chicks for a week or two as their sole enclosure. A box that small would also br prone to overheating unless there were only one bulb, which would be a danger because they would be without heat if the bulb failed during the night, or while you were gone. Since this seems to be where you were heading in your message, I would urge caution, and perhaps rethinking. I intend to rotate them so as to keep the grass etc. growing. This is a private posting area.