I made a dual-muffin fan system to vent the closet to keep it cool. I attached it to the light and it actually vents the light some and it is able to be put closer to the plants.

The setup is two 3" muffin fans that are pushing air the same direction are attached to the end of a small section of 3" PVC. All of the spaces are sealed so the fans can move the greatest amount of air. On the inside of the PVC is filter material which absorbs the fragrance of fresh cannabis (we tested it by blowing dank hits through it and we couldn't smell anything on the other side.

After it is attached to the light, a 3" flex ducting line is attached and the air is vented out.

Another choice would be to cover your closet with a thick blanket and point an opening away from the window. This would keep you stealthy and cool. Or even a combination.