You might do two things for the window, #1 Hinge the cardboard so it stores more easily. #2 use darkroom/blackout cloth to cover the cardboard. I use foamcore covered w/cloth & the flexibility of the cloth will compress to fill small nooks & crannies..
Vent? how 'bout using a section of flexible aluminum venting similar to dryer vent. Could paint interior black to cut down light from traveling through & give it a 180 degree turn or two somewhere in its travels.