Hermione hadn't thought of using the chamber as Bellatrixs prison right away. She had first thought of trying her luck with the room of requirement but of course, she quickly recalled how Crabbe had set the place on fire with that curse and how it was likely the room had been utterly destroyed due to the power of the curse. Hermione's next thought had been of the shrieking shack no one ever ventured inside it and it was a good spot far out of the way of anyone and unlikely that Bellatrix would be stumbled upon by some unlucky passerby.