Here I start to remove all the old components in the Apple G4 Cube. You can see that there isn’t really much room for air flow, will be a tight fit for anything.
All the airflow will be coming from the bottom and coming out the top.
Will be pretty much 3 channels for airflow, CPU+Motherboard, PSU, & GPU channel. The CPU & GPU channels might need fans on the bottom of the case to help move the air since both of them will have side mounted fans. But time will tell, maybe the fans on the top for outtake will be enough to direct the airflow.