UPDATE: I've updated the parts to fit 5mm magnets. There are a separate set of parts that were labeled 5mm.
In response to several requests to make assembly and disassembly of my E3Dv6 duct (http://www.thingiverse.com/thing:494430) more easy, I've updated the duct and eliminated the bolts that hold the 40 mm duct to the extruder body. I've redesigned the E3D shroud to be a bit tighter with sockets for two 1/4" magnets. These can be easily found on ebay and Amazon. Once the magnets are glued in place, the ducts snap together and hold very tightly.
Because the 40 mm fan duct attaches directly to the 30 mm fan shroud, this design is no longer Makergear M2 specific but a more general design. This too is to address a series of requests for a more generally applicable design.
I've also posted a mirrored shroud so the 40 mm duct can be mounted on either side of the extruder. The mount is centered on the 40 mm duct so this duct can be fitted on either side of the shroud.
I've printed these with both 0.3 and 0.4 mm nozzles. They should not require supports if the files are printed as oriented.
I've been testing this update on my printer for the last month and see no change in performance, other than it is much easier to disassemble / reassemble. Because of the efficiency of the design I suggest 20-25% fan speed.
Once printed glue 1/4" magnets into the sockets. Take care to properly orient the magnets in the sockets so that the parts attract rather than repel each other. There is a small tab under the magnet socket on the 30 mm fan shroud that should be snapped off and trimmed.