Add a fan to the side panel case window:
1) Make sure the window material is sturdy enough (thickness) to support the fan's weight.
2) Make sure the window is firmly attached to the side panel, commonly with rivets, screws, or strong double-sided tape.
3) Will the fan be intake or exhaust? Plan the fan's location: at GPU level, a little lower, or a little higher?
4) After deciding 3), use the fan as the template for marking the screw holes and air-opening.
5) Re-measure and re-check the template on the side window.
6) Cut the air hole.
7) Drill the screw holes. Or, if you would rather use double-sided tape instead of screws, you won't have to drill screw holes, and the tape will serve to dampen vibration and noise - use transparent tape, and don't plan to remove the fan once it's installed.
8) If you use screws to attach the fan to the window, be prepared to make and use a thin gasket to put between the window and fan to dampen vibration and noise.