Based on loco's E3d v6 40mm Fan Duct
I modified the model to be snap on, even in PLA, as well as adding zip tie tabs to be used either to more securely mount the fan duct, for wire management, or anything else.
Caution: I have yet to test how durable the PLA is over the course of time, but if I have problems, I will report back. However, even with the hot end facing towards the front, the shroud should not touch it, so direct melting shouldn't be an issue.
If the shroud fails to slide on, be sure lip is riding the top fin of the heatsink. Failing that, try flipping it over the other way. Snapped right on with my Hatchbox PLA, but if your PLA is too crispy and brittle, it may fail. Tight fit, so may need a well calibrated printer. If the print is still too tight horizontally, scale up to 1.01x.
May need supports if you want the zip tie bridge to look pretty, but it is thick enough so a little sagging shouldn't matter. Also, I arched the bridge to minimize sagging, so I wouldn't even worry about supports unless you are OCD about those things.
Edit: Improved the model so that the zip tie tabs are more rounded and look nicer, and tapered the lip so that installation is easier when using stiff/brittle filaments.
Print fan mount side down.