There is supposed to be a plastic window type thing in there, with vents pointing outwards and downwards, its a one way vent meaning that air can get out of the car but not get in.
cabin road noise will also decrease by installing the vents, right now its just an open hole and it will also cause dust to enter the car interior at great pace along with loads of HOT air that will heat up the cabin.
Suzuki still sell them brand new but only at the dealership parts department. Every car has them - and suzuki have a few that use the same parts for many models.
Also get some thick plastic from a bookshop and some sticky black tape, paste the plastic to the door metal panels before installing the door cards.