There are many fan shrouds available on thingiverse, but lawsy's version is probably the most highly recommended. Replace the "jigsaw" extruder assembly first, and then mount the fan shroud as an accessory.
mk5 extruder: http://www.thingiverse.com/thing:111213
fan shroud for mk5 http://www.thingiverse.com/thing:115659
Lawsy has also uploaded fan shrouds for the mk4 and mk3 extruder assemblies.