Let me see if I have this right. You are in Maine, you have some 8 week old cochin, you have a room with a concrete floor and insulated walls, but with some windows. You want to know if you can keep your cochin in a 6x6 plastic enclosure in this room over the winter.
At 8 weeks they should be fully feathered out. They should not need a heat lamp. I appreciate that they have been kept in a warm environment and you do not want to shock them by moving them from a 70 degree Fahrenheit life to one that will soon be around 0*F at night. If you can gradually lower the temperature, they should be fine without any heat.
Is this room atttached or detached? How cold will it actually get in the room? I'm not clear on that from your post. I think it is an extension to your house so it will not get all that cold in there, but it could easily be a totally detached room somewhere else. What you are looking at is how cold it will get in there, not outside. In either case, if the room is air tight enough to prevent the wind blowing though, it should not matter as long as you are not shocking them by moving them directly from a semitropical environment to an arctic environment.
Another issue is ventilation. They are going to put off a lot of moisture, both from their poop and their breath. If their poop builds up, it can put off a lot of ammonia which is bad for their respiratory system. The room needs to be vented well enough to get rid of the moisture and ammonia. A decent poop management plan will help too. If it is an attached room, it will probably not get cold enough to cause frostbite even with high moisture, but you might get condensation on your walls and ceiling. In any case, you can expect a lot of dust.
I'm not sure how many chicks you have, so I can't comment on whether that is enough room. I'm also not sure what you are trying to achieve with that 6 x 6 plastic enclosure. I can see some possible advantages, draft protection, trying to contain some of the mess, keeping heat in if you want to add heat. The room is going to be real dusty but I can see trying to keep the heavy duty cleaning down to a smaller area.
I don't think heat is going to be much of a consideration to their comfort. I think space and ventilation will. I don't know enough top further comment on those.
Good luck. Sometimes these things can be a bit rough to work out.