Inspired by a4lg’s post, I decided to make a small 3D printed mount (Thingiverse) for NF-A4x10 FLX 40mm Noctua fan. The mount itself is pretty boring but it’s designed in a way that everything should tight fit (no screws required to mount):
This was printed using PETG which has melting temperature around 75°C. For a prolonged use, ABS is probably a better material (but it’s harder to print, and I assume heatsink won’t ever got that hot).
The first version of the design that I made create a transitional shape between 40mm and 20mm, but this resulted in a pressure drop and is very inefficient in cooling the CPU. The current design is not ideal either, but it managed to get similar numbers as with a4lg’s post.
It’s worth noting that this approach requires 120mm chassis fan as an intake. Fortunately, silent 120mm are much easier to find. In my setup, I use NF-A12x25 (120mm x 25mm) as an intake fan at the top and NF-A9x14 (92mm x 14mm) as an exhaust at the back. Temperature during normal usage (not stress) is about 34.6°C (temp1)/41.8°C (temp2) at 25°C room temperature.
Measured noise of the entire machine is 36 dB. Given this machine currently sit next to my bed, I won’t mind leaving it running 24x7 at this noise level (e.g. air conditioner is much louder). Overall, I’m very satisfied with the setup now. 