We built what we call our chick U.. having fostered preemie babies from the NICU we found the term CHICK U to be appropriate.
Under our raised coop we completely closed the the outer sides with the wood covering the upper coop. Then we made a chicken wire door the width of the coop. It's basically a short version of the big coop upstairs.
It's a somewhat short space. We have little roosts etc. And we do use a lamp.
This space is basically secure inside the large coop/run area for the other hens. They can see one another through the wire, but can not access each other.
I find my "turkey Dirk" watching over the chicks from his side. He's a gentle protector.
We have used this to set our broody hen to hatch and raise her clutch, and we've used it for our new chicks two seasons now. It works well and keeps everyone somewhat in the same area but secure. The space is about 6x4 ft and about 2 ft high.
When we aren't using it for chicks, it remains open for extra space for the hens.
I'll see if I can find a picture.