I understood the design before you replied. I still think I'd try a 92mm fan on top before I got much fancier. I don't know about you but I'd borrow an intake fan off a Solo or use a spare 120mm fan I have laying around to see how well the concept worked. If you don't have a fan laying around that you can test with I can see being more reluctant to try.
I would use the Scythe 92mm Case Fan - DFS922512L 2000 RPM at 12v but use speedfan to lower the RPMs as far as I could. Of course that fan has been out of stock on Jab-tech for a long time so you may not be able to find one.
YATE LOON 92mm Case Fan - D90BM-12 is in stock and is even cheaper. It's 2300 RPM at 12v but again I'd speedfan it down a chunk. If you prefer to hard wire the fan it is rated to work down to 6.5v so 7v fixed would work no problem and 5v might work depending on luck of the draw. And you can get them as cheap as $3.50 + S&H.
Your HS is one of the AVCs below. All of my AMD CPUs have come with the Ajigo instead.
Heatsink/Fan Noise Fan V Â°C Rise Â°C/W MP Weight
Ajigo MF064-074 (70mm fan no pipes) ($ 5) ?? ? ?? 0.?? 373 (363+fan)
AVC Z7U7414001 (70mm 4 heat pipes) ($10) ?? ? ?? 0.?? 440
AVC Z7U7414002 (70mm 4 heat pipes) ($10) ?? ? ?? 0.?? 440
If you want to keep the weight down and get a new cooler you could consider
Heatsinks Compared at ~23 dBA@1m:
Heatsink/Fan Noise Fan V Â°C Rise Â°C/W MP Height Weight
Asus Triton 75, Nexus 120 ($45) 23 12 18 0.24 90/115 473* (350+fan)
Thermalright XP-120, Nexus 120 ($??) 23 12 25 0.32 63/88 493* (370+fan)
Thermalright SI-120, Nexus 120 ($??) 23 12 ?? 0.?? 90/115 523* (400+fan)
Thermalright SI-128, Nexus 120 ($45) 23 12 21 0.27 92/117 633* (510+fan)
Thermalright SI-128 SE, Nexus 120 ($60) 23 12 ?? 0.?? 92/117 633* (510+fan)
Scythe Andy with stock fan ($45) 20 12 16 0.21 100/125 685 (???+fan)
Scythe Ninja Mini, stock fan ($40) 28 12 19 0.24 115 678* (580+fan)
Scythe Ninja Mini, Nexus 92 ($??) 20 12 ?? 0.?? 115 678* (580+fan)
Scythe Ninja Mini, 92mm fan ~23dBA($??) 23 12 ?? 0.?? 115 678* (580+fan)
Height is heatsink/combined height or if centered the fan doesn't add to height. For the ones with a / you will want an additional 25mm or greater clearance for airflow.
lists the Nexus 120mm fan at 123g
If you already have a 92mm fan laying around the Ninja Mini has promise as a good choice. If you don't the Scythe Andy comes with a fan that is worth using. If you are really adverse to the weight you could consider the Asus Triton 75 it doesn't cool as well as the Andy but it's lighter by a noticeable amount.
Since I'm in the US I just grab sleeve bearing fans by Scythe or Yate Loon that are nearest the rpm of a Nexus and I usually do so buy buying a few at a time from sites that sell them around $2-6 each plus S&H.
120mm Nexus = 1000 RPM
92mm Nexus = 1300 RPM
80mm Nexus = 1500 RPM
fwiw the 70mm stock fan on my Ajigo is running at about 1300 rpm as I write this simply because the motherboard I'm using it on won't let it spin down as far as I like. On other motherboards I've had it under 600 rpm with speedfan. The 12v speed on that fan is 3000 RPM so it isn't unreasonable to assume that you can slow a fan down to 1/3 or 1/5 of its max speed without spending money on a fan controller.