You shouldn’t need a fan if you are just converting the systems into a display. The natural drafting (like a chimney) is already present with the gaps at the bottom and at the top as the heat will rise up and out.
If you remove the Bluetooth antenna and Apple Logo you will offer a short path for the air to travel from the RAM cover with or without a fan so the rest of the case won’t gain enough cooling.
A good way to see this is in your kitchen sink with water.
Take a flat cookie sheet and place it at a sharp angle down from the slightly open faucet the angle will effect the waters width of spread so as the angle gets shallower the spread is more!
Air flow is really no different. But, clearly the slow flow of the faucet can be equated to the volume of heat so if the system was really running hard then a fan would be required (hence why your system had one) but as you pulled most of the heat sources out the need of a fan is really not needed here. Just like how a harder running stream from the faucet would need a bigger drain.