I'd recommend partitioning off a small area for her within the hen house. A 2 x 3' area should be fine, with a nest set up for her on the floor.
If other hens continue to have access to her next box, they'll lay their eggs there as well, adding them to the clutch. This means you'll have to mark her eggs at the beginning of incubation and check the nest every single day to remove the eggs that other hens have added. The other risks include eggs getting broken as multiple hens come and go from her nest box, and the broody hen getting into the wrong nest box after she's taken her daily break, which may kill the developing embryos in these cold temperatures.
Edit - I forgot to mention that I only keep the hen and chicks isolated (but in view of) the rest of the flock during incubation, hatch and for 4 days after the hatch. Then I let the mother hen and chicks mingle with the rest of the flock. The hen will protect and integrate the chicks into the flock, saving you LOTS of work.
Edited by song of joy - 10/19/15 at 5:35am