This fan duct is designed to be mounted on a 40mm attached with ReprapPro fan mount.
A fan duct is particularly necessary when printing ABS since high bed temperature means high thermal dissipation which can exceed the heating circuit ability to maintain proper temperature. This can result in your bed temeperature decreasing too much. With this duct, air is directed only near the printhead: thermal dissipation is limited for the first layers and once the printhead is a little bit higher cold air does not even touch the bed.

This version has many advantages over existing designs :
-It is fixed with the same principle as reprappro fan mount (tight fit in the fan holes) so that installation is very fast. Take care though, when removing it from the fan, not to break the pins (insert a blade between the two part and rotate it).
-Its open design also means the fan will not stall because of reducing cross section in the duct.
-No more clearance than the bare fan is needed

The only downside is that it is a little bit more challenging to print, though with support material included in STL it should be ok.

It is easier to print if you have a cooling fan since there is a bit of overhang.
You can put some aluminium tape at the exhaust to better orient the air flow.

CAD speaking, the upper part of duct is cut at the end of its construction. If you need different upper profile, you can easily modify the line of cut in catia.