I found that all of the circular fan ducts for the Anet A8 are angled/off-center from the fan output to the circular section. On my Alunar M508, the geometry between the cooling fan output and the heater block is straight-on, so round Anet A8 ducts end up off-center on the heating block.
Since TabLeft was so thoughtful to include a link to his Fusion 360 file (thanks!), I could easily modify it to better fit the Alunar M508. Besides a straight and center downward duct, I also moved the nozzle connector (where it meets the fan) -4mm (inward) to better center on the heater block.
Here's the modified Fusion 360 file: http://a360.co/2DIY8sp
2018-03-12: Added a shorter version (labeled "21mm"), as I had to tweak the geometry when I replaced the threaded throat on my hotend. It ended up being screwed into the extruder housing a little further. If you measure from the bottom of the 50mm fan to the tip and it's in the 21-24mm range, this one should work better for you. The original is for longer distances... it all depends on how far up you've screwed in the extruder/heatbreak threaded tube.
I had to lightly sand the part that goes into the fan... it is a tight fit.