I've built 3 cocktail cabs (3 control panel type) and I've found that the best airflow was to cut two big holes in the lower front covered by speaker grilles, mount your speakers underneath instead (requires your cab to be on legs/castors etc), and cut a large hole, cover with a fan grille then mount a single 120mm fan on the upper back. This catches the hot air from your PC and also pulls air across the back of your monitor, extracting the rising hot air out the back.
You also get the happy effect of the sound coming from the center when you switch to horizontal gaming.
Note, this is for a cab with speakers ordinarily on the front