i created a fanduct for the noctua 14cm fan and want to share 
fan duct 140 mm 7.5mm.stl (814.3 KB)
this is the one you see on the picture 7.5mm .
i also made a version 1mm higher but i didn’t tested / printed it yet →
fan duct 140 mm 8.5mm.stl (814.3 KB)
I used that fan for a while when I first got the desktop and I was surprised by how well it worked. If you look at those photos, the back panel even has a cutout to precisely match the fan’s outer diameter. And it fit right on to the mounting pegs with no force at all. But, I am now using the provided 120mm HS fan as soon as I bored it out to fit. I didn’t measure much difference in processor speeds with it, but I also didn’t measure the noise.
I was thinking of asking Noctua to instead create a version of their side panel that matches the larger 140mm diameter.
Another idea for a shroud on this fan would have been to put one under the fan so that it directs all 140mm of swept area into the fins. As it is, the fan is blowing past the radiator on a couple of the sides so we won’t get all of the benefit of the large swept area.
I love projects like this, and if I had an A14 around I’d totally print it…the caveats are:
As you mentioned the heatsink not being quite big enough for it
A14s are lower RPM that 120mm fans as a rule. Partularly the stock 2150RPM stock A12.
While the A14 fans move more CFM or m^3/minute…the issue in a 5L SFF chassis isn’t air flow, it is static pressure. Because the HSF is both the intake and has to push air out the case (AKA overcome restriction). And larger fans just have lower static pressure.
2&3 are why Noctua in their testing recommend the stock G1 A12 fan over the newer G2. The Stock fan has both a higher RPM and a higher static pressure. Now, both the A12 G2 and A14 would be quieter by a bit–but whether that is worth the thermal trade off depends on your ambient temps.
I was using a 120mm fan before. But the fact that it extended beyond the heatsink was exactly the reason I switched to the 140mm one! As you can see in the photo, I’m using an SSD with a heatsink, and it was getting dangerously close to 70°C, the temperature at which it starts throttling. The 140mm fan runs at a minimum of 10%, is inaudible, and the SSD temp is significantly better now.
btw the airflow in this case is better than you’d think. I tested it with my vape.
PS: I like the deep sound of the fan at full speed, and the CPU stays just below 80°C under full load.
