Rule of thumb? Usually an intake fan at the bottom and a vent at the top (on opposite sides). If you're building a panel for a specific site, its environment will dictate fan/vent locations ie dirt, dust,mist etc...If you think an inlet and outlet fan is required - panel cooling is critical - I would install an air conditioner of some type. The usual practice is to have one interlocked with both a thermostat (running only when it needs to) and with the panel door(s)...so that if someboby leaves the door open/ajar, its not trying to cool the entire building....don't laugh, I've seen it!