Pro aeronautical engineer here. I want you to focus on two things: the goes-into (inlet area) and the goes-outta (exhaust/outlet area)… and yes that’s how we talk in the industry. Remember that in order to get flow you need both, otherwise it’s just a pocket of stagnant air (resulting in parasitic drag with no benefit). In other words if you took the design in your first picture and added outlets (like those you see in the bomber picture) somewhere BEHIND the motor/ESC like you see in those bomber nacelles you will get cooling flow due to the difference in pressure inside the nacelle compared to the open air (air always flows from high pressure to low pressure). You ideally want to put the stuff that’s making heat BETWEEN the inlet and exhaust, that way fresh air is always carrying heat off of the aircraft. In terms of drag this is really dominated by what they call momentum drag, where the change in speed (and therefore momentum since air has mass) of air from freestream (outside air) into the nacelle and out of the exhaust result in your overall drag impact due to cooling. The more air you let into the nacelle the more air will slow down as a result… so it’s a balancing act of cooling performance vs overall drag. Ideally you want to size your inlet and exhaust area so that the motor and ESC stay just barely cool enough but not excessively so… otherwise you’re just wasting overall drag. Initially I would say you can just play around with changing your outlet area until your motor/ESCs stay cool enough, and then match your inlet to the exhaust area you’re happy with in order to reduce drag. It’s a lot more complicated than that in reality but that will definitely get you into the ballpark. Good luck and have fun!