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Remember, you need a lot of fresh air exchange to insure tghat your plants have plenty of CO2, temps between a low of around 60F and a High of 90F, and hopefully you can keep the temp within a 15F range for a 24 hours period, lots of the proper light at the proper time, water and nutrients to grow MJ. You need to balance the effect on all of these factors in deciding how to deal with any particular item. IF you can do it, use one thing to handle more than one issue at a time. For instance, you can use your window covering to deal with light leakage, temp control and fresh air all at the same time if done correctly.
My first thought is that you need to separate your intake and exhaust openings a lot more than they are. A 6" fan will probably draw all the air that it will exhaust in under the door if you have at least at 28" x1.5" gap between the floor and the bottom of the door without any problems of sucking the hot air back in from your exhausted air. If you have any HVAC ducts into the room, the exhaust fan will also draw air through them, so you probably only need an exhaust opening at the window but I think a 4" one is probably too small, especially from your experiences so far.
Also that ducting is really cutting down on your air flow. Try this. Disconnect the ducts, cover the holes, open the bottom of your box up and mount your exhaust fan in the top area of the window. Then turn your light on at night, go outside and check the window and see if it looks like there is any more than just a night light on. If not your light control is okay. If it is, then try to paint the inside of the box flat black to help keep the light from reflecting around inside of it. Then try your temp check in the day light. The absolute top that I would consider for the temp on the plants is 90F and that is with plenty of breeze generated by an oscillating fan.
Then comes the really bad news, you need to find a way to confine the light in your growing area because 1000 watts is going to disappear in the rest of the room without having some kind light confining method. Cover the cardboard with mylar film, shiny white Panda type plastic or at least paint it with white -- not off white, eggshell or anything like that, in my opinion I use semi-gloss white or high gloss but some here say to use flat, but we all agree on the whitest white you can find-- just pure white paint.
Get yourself a good grade of poly tarp and put it down on the floor. If you can get it, the silvery colored stuff is good, but in my opinion protecting the floor is more important than reflecting light off of the floor.
When I started I was trying to do the same thing in a 13' x 10' room and it really sucked even though I was running 3 400 watt lights. It looks like you are at least growing in the corner which will really cut down on light loss.
I ran a 6" high volume commercial fan exhausting through the boxed off window area much like you are doing and in the Summer with the central AC for the house running, I could keep the temps down to around 90F which is on the upper side of acceptable. To lessen the problem of light leakage, I set my lights to come on at 5 AM and go off at 9PM in veg and on at 8AM and off at 8 PM which are very reasonable hours for my household to be up. I moved my operation to a similarly sized downstairs bedroom in a split level house and not only drew the fresh air in from near the cool concrete floor but exhausted it back into the rest of the house from about 4' off of the floor. It also made hiding the odor easier because it wasn't actually vented outside. I have split the downstairs room into a 6' x12' flower room and a 4' x 12' veg room and it is working real well.
I hope this gives you some ideas and don't hesitate to ask more questions.
Oh, try to get it right the first time, because when you say I'll fix it later you will find later never comes. At least that sure is the way it works for me. I waited to paint my downstairs room and wound up using it for over 6 months before I got it empty enough to actually finally paint it all white. And I'm still trying to get the veg room cleaned out enough to paint it.
You are on the right track and have a great area to work with in comparison to what a lot of people have to work with.
Great smoking!
