We used a zippered dog pen last year. It was left over from when we had a dog recovering from surgery. She needed to be contained but also part of the household for 6 weeks.
Plastic, then cardboard on the (wood) floor were placed underneath, pine shavings inside. Set the whole thing under my seed starting table in the dining room. The (7, early April) chicks were safe from the curious cat and dogs; we could monitor and be amused.
The zippered top allows for easy access to food/water/clean up tasks as well and for handling. A seed starter mat was used for supplemental heat the first while.
Chicks got big fast and needed more room at about 4 weeks. We got creative.
In late May, they went outside at 5 1/2 weeks and did fine. They seemed delighted to be on grass. Every year is different here. We watched the temps , esp overnight and acclimated them slowly.
https://www.amazon.com/ESK-Collecti...d=1520699485&sr=8-7&keywords=portable+dog+pen
Eventually I hope to have an outdoor brooder, but I will brood inside again this year - It's the best way to assure the exuberant Catahoula adopts the chicks as hers. Will use same set up this year but in the side room instead of the main part of the house. And for only 3 chicks. HAH good luck there, eh?