put a couple of fans at floor level.
i made extra air vents from the blocked off window.
first build a 2"x2" wooden frame around the window.
leave the window open about 4", leave the curtains up, just close them.
leave a 6" gap in the wooden frame where you wanna run your air vent.
fit two 2"x2" wooden frame all along the wall, butting up to the gap so you've now got a 6" channel, (make it as long as you like) cover the end up.
then fit 4" battons every foot or so in the channels (first one against the inside top , next one up against the inside bottom one)and so on, so the air comes through a kinda 's' 's' 's'chicane.this also stops the light coming through.then cover over the top with the same wood you covered the window with.( filler any gaps)
hope this makes sense? this may also cure negative air flow problems.
i've built many growrooms over the years, always glad to help fellow growers.
if it don't make sense maybe i could send you a diagram?
when you fit your charcoal filter( suspended from ceiling by chains?) you can cut a hole in the window board to take the exit tube.