You don't have to build anything. Get one of those inexpensive wire dog crates--bigger than small... smaller than huge. (I bought a small one for hospital lodging at Walmart.) You can make some kind of privacy covering if you need to, but don't make it stuffy esp as it's summer.
Put it in the coop where she's sitting now, or as near as you can. Stick some bedding in there, some food and water, and put her in with some fake eggs to make sure she doesn't freak out and damage your pricey eggs. If you need to, you can start them in the incubator (which you ought to have, just in case, anyway) and give them to her at night once you decide she has settled in.
If you decide you want a designated broody coop, here's one I made.
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