I agree with Bugcrusher. It looks like you've built a very nice tight coop. In Jersey, unless you have something like a silkie bantam, a comfortable coop, free of drafts, rain or snow, is just what laying breeds need to be happy and healthy through out the winter. I have used supplemental heat (in SW PA) only a few times when the temps dipped into double negative (-15 F). And heat lamps in a coop full of clumsy clucks gives me nightmares. I used an electric oil filled register type heater and set the thermostat at the lowest setting. If you get the ambient temp back to just below freezing your birds will stay acclimated to the cold but get a nice reprieve from the most brutal of temps. Most reg. and large body fowl (with standard feathers) thrive thru the winter months in freezing climates easily with shelter, unfrozen water and food. A well feathered hen scratching around in the snow is one of my favorite scenes around here in the winter months.
As far as a huddle box goes. The window over your roosts poses a problem with installing something permanent. In order for you to design effectively you have to examine the function of what you would like to achieve. So, you would like to build an area in your coop that will provide a space that will maximize your chickens ability to keep themselves warm. You are already free of draft and moisture and you already have a good roost area. Chickens create their own heat, heat rises so if you want to maximize that heat, cover the roost with a low enclosure. even a drape of some kind; just don't make it too high or the heat will dissipate before it hits the top and fills the space. A height of 2 ft. would be plenty for large body birds. Attach a curtain to the wall above the roosts, throw a couple of cross pieces, wall to wall, above (24 inches?) and parallel to the roosts. Drape the curtain over the cross pieces. The cross pieces could be any board lumber or rods. Whatever you put in, don't make it too high and make sure to leave a lip (4"- 6") hanging down into the front opening, from the top of the enclosure, to trap the heat a little.
You have a nice coop so don't worry too much about your clucks. Think of what temps you could survive if you were wearing heated boots and a feather filled body suit!
Good luck with your flock. I know, I was too long winded :) Polly Kate