I installed the 40mm fan and made a duct out of clear blister packaging plastic I had on hand. I worked out a flat pattern first, then transferred the outline and cut it out and folded it up.
Then I used clear packaging tape to tape it closed where it overlaps. I trimmed the duct to fit with the fan mount bracket and the extruder’s heater block.
The duct is trapped by the fan mounting screws at the top and small wood screws (maybe #2’s) from the inside with a washer under the head and screwed into a smaller hole into the duct tab.
Making this duct by hand was quicker for me since I have done this type of prototyping before. Plus it is crystal clear and easily trimmed. By the way, in case you are curious enough to ask; it was a GE water filter package, maybe .2 mm thick? I did not measure, that is just a guess and its thickness is not critical. Just don’t get it too hot, since it is thermoplastic.
Here is the flat pattern of the duct, note the dots are where the screws wound up to attach it to a 40mm fan, so adjust the tabs when you cut them out to give more edge distance.