Yeah a lot of people use that corrugated stuff, it sounds nice! Myself, I have a 10X20 foot dog run for my 7 birds (5 hens and 2 ducks) and I have tarped the top and most of the end sides of the whole darn thing with heavy duty haystack tarping and bungees. The roof of the run is peaked in the middle so the water runs right off. Where are you located? I live in Boise Idaho and it got down to 20 degrees for a couple nights last week...I think the first night it sort of shocked them to know how cold it can really get LOL this is their first winter, and my first winter with chickens so I understand your concerns. But they seem to be doing great.
The trick is keeping the water de-iced and like everyone else said, the drafts out while still providing enough ventilation that CO2 that builds up from the chickens breath and poop can escape and frostbite doesn't become an issue. Someone once told me that any sort of ventilation that isn't causing air to run straight at the chickens while they are roosting is sufficient, you just don't want any wind-tunnels or open windows that are going to be right at chicken level or below. Your coop is very cute! I have a small coop as well and my girls keep plenty warm huddled up together.
As far as lighting or external heat sources, I frankly think they are expensive and a waste of effort for many reasons. Coop fires do happen from lack of sufficient wattage allowance, wood or straw becoming too hot etc...chickens used to extra light or heat will ALWAYS be used to extra light or heat so if the heat fails one night...they are likely to be VERY uncomfortable than if simply allowed to build up their natural winter coats. Chickens survived the cold long before electricity was invented. I hear stories of folks on here with chickens in Canada and Alaska who do not light or heat that do just fine. Supplementing light will ensure that your laying hens continue to do so throughout the winter. Chickens need around 14 hours of daylight to lay, but this will also burn them out sooner. Winter is a time for hens to relax, moult and put their energy into keeping warm. I don't like to mess with nature. Also, many people say their first year layers continue into winter despight the change of daylight hours anyway, and this has proved true with mine as well (except one who has decided to moult) Sorry this is a bit long, but I hope you find it useful. Stay warm this winter!