I didn't really like the look of the R3 fan duct (Shown in orange). It looks very focussed on frontal flow, rather than a surrounding flow. However, I did like the idea of tilting the fan over for a bit more power. This is a design similar to robrps's nice design: https://www.thingiverse.com/thing:2808683

It features simpler and more minimalist ducting to increase the throughput.
It has ducts diverting around 1/3 of the flow to each side, as well as 1/3 from the front.
It mounts in a similar fashion to the R3 fan duct.

It is untested at this stage. I plan to print the updated extruder body in a week or so and I'll test it at the same time. There's a decent chance I'll need to trim a bit from the right edge of the fan socket, however I need to pull my fan off to check those dimensions.

At some stage I'll probably do some more fillets and other niceties. Maybe try and find somewhere to write "HOT!" on it.

Update: Added a second (untested) mount (shown in green) that adapts the fan to both original mount points.

Update 2: Big update. I did some testing, and the Prusa R3 duct was actually most effective out of the ducts I tested. I suspect this one is pointing a bit high, whereas the prusa duct now directs a lot of flow downwards. However, I didn't like the way the Prusa one attaches to the machine. Therefore, I've modified the Prusa R3 duct to fit with the double mount attachment developed here. I've added the following files:

Fan Shroud _Prusa R3 extended.stl - This is the standard R3 duct, extended to better fit the fan and compatible with the double mount adaptor. It's a tight fit (though actually not as tight as the Prusa R3 duct).

Fan Shroud _Prusa R3 extended_Relaxed tolerances.stl - As above, but with much more relaxed fan tolerances.

Fan Shroud _Prusa R3 extended_Relaxed tolerances_PC.stl - As above, but I've also trimmed off the vertical walls that stick out the front. This is for prints that use a brim(eg: polycarbonate), as the brim can be quite tricky to remove from the single walls.

Fan Shroud _Prusa R3 extended_Relaxed tolerances_PC_Improved Bridging.stl - As above, but the roof has been sloped. This eats into the duct space a little bit, but also should improve printing of the roof.

Update:
Fan-Shroud-_Prusa-R3-extended-netfabb.stl is a version of the R3 extended duct that's been run through netfabb. Slic3r and Cura seem to be fine with these files, but S3D is fussy. :D

Update:
Files run through Netfabb. They should all work in S3D and other finnicky slicers.

Update: Step files added.

Update: The revised R3 files have lost the "arm". This makes it hard to mount the old style fan, and obviously to use the double mount and new shroud I've developed. A variant of the R3 extruder cover with all the goodies from the R3 design, plus the arm has been developed here: https://www.thingiverse.com/thing:3032939