Here's a picture. It worked really well. If I brood chicks (ie, don't have a broody hen), I will do it this way again. You can just see the heat plate on the right side of the crate. And cute chicks in the mirror.
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You can see that the cardboard is bent/wrapped around in the corner. It's just round enough that the chicks can't get "stuck" there.
I had some 2x3 welded wire fence around one side of the crate, with some sheets draped/clothes pinned to it. I would sit in there, open the door to let them out, and have some play time.
The left side faces a window. That helped them get used to the natural dawn/dusk cycle.
The crate was about $90, so not cheap, and that was 3 years ago. I looked at it as a one time expense, and I have more uses for a dog crate than just brooding chicks. I use it to harden off plants for the garden (minus the cardboard). Critters can't get in, and I can drape sheets over the top for warmth and to adjust the amount of sunlight.