I would look for a cage with a fairly thick gauge wire base, preferably with close-knit wire 'holes' (rather than big wire 'holes'). Sorry I don't know the technical term, but hopefully you get the idea! If the cage has close-knit holes on the sides as well - even better. You can then use it to house Momma and her chicks, and the chicks won't be able to escape the cage through the holes, leaving Momma trapped inside!
It's also important that the crate is fairly spacious - a broody hen or a sick hen will still need room to at least extend her wings and have a bit of a flap about while she is in there.
To break a broody you will also need to put the cage up onto bricks or something similar, to allow the air to flow freely underneath the cage and the hen. This cooling action on the rear end of the bird is what will break her broodiness. Keep in mind the weight of the cage will need to be what you can comfortably lift up onto bricks.
Obviously, if you have a sick bird (rather than a broody one) you will want to pop the cage back down onto ground level and fill it with some wood shavings to make it comfortable. In hot weather you will also want to move the cage into shaded areas etc. It must not be so big that it is not moveable. You should allow enough room for water and feed containers, and for the bird to stretch though.
On a practical level - look for a nice wide door on the cage, to allow easy access to the hen inside. It may be with a sick bird you will have to get her in or out of the cage often for treatment. You don't want to be trying to squeeze a big puffed up bird through a little door!
That's about the best I can come up with. Hope that helps!
I use a large pet airline type crate, making sure that the door grill openings are small enough to keep tiny chicks inside. In the line of bad experiences teaching truths; Small crates will allow a raccoon to reach through the door to kill babies and the hen. Wire floors ditto, or at least toes removed. Ugh! Bantam chicks are TINY and can escape through chicken wire and some dog crate wire doors.
Also, dust the base with permethrin or something to prevent the broody from dying of a mite infestation gone wild. Mary
I only need one for a safe way to confine a sick or unwanted broody hen and cage will be placed in a protected coop/run or in garage. So predators nor baby chicks are not an issue.
Planning to buy either a dog cage or a rabbit cage that can be folded and stored when not in use…dog cages offer more height and stronger wire structure, but have larger spaces in the wires. Rabbit cages have closer wires but a maximum Height of 20". Probably either will suffice, but I want to make sure I get one that will not cause injury to my girls.
My concerns: wider spaced wire may let chicken poke head out and get hurt? Rabbit cages 1/2 inch type wire on bottom could abrade feet on heavy BO's.