These are all a work in progress, so please print with care. You may need to bodge these together, or take care when using. The 40mm fan mount is the most tested, and the 40mm fans are the easiest to source.

A quick run down / rambles on each design:

40mm fan -- Tight wiring, fiddly assembly, duct angle?? The ducts can be printed a few different ways, but I suggest printing with duct facing upwards to avoid need for support. Will require 4x M2 screws for the ducts, and 4x M2 screws for the fans themselves. I'm not sure on length as I cut down my own screws using a dremel, but I can find out if anyone would like. Main issue -- 40mm fan design is awful. The blades of the fan stick out of the enclosure meaning that a lot of air can escape out the inlet, rather then exiting the duct. I believe the 30mm fans will fix this

30mm fan -- Utilizes 5v B0503AFB2-8 fans. untested at this time of writing. difficult to source 30mm blower fan. Fixes space issue, gives more space for wiring. Ducts require support to print. Fans must be glued to body. I may use screw fixings, as the fan duct is already designed to be screwed on, but this comes at the compromise of less screws on the cold end fan...
To me, this design has the most promise.

Pipe mount -- Solves wiring issues completely, more compact and more space. difficulty in bending pipe, and using a powerful enough air pump for required air pressure. I suggest using 4mm copper pipe for the duct attached to flexible silicone pipe. 4mm copper pipe is easy to cut and bend by hand, cheap and easy to source.

General thoughts --- Having slaved over fan design for far too long, I feel like I can confidently have some input into this area.

Managing micro-air flows is difficult. If the two ducts meet each other head on, they can actually create a bit of a deadzone as the two air flows 'cancel' each other out. Positioning the angle of the ducts too high causes obvious issues with the hotend, and placing them too low can mean the correct area is not cooled enough and can also direct flow down the side of the print and directly into the heatbed (also bad).

It seems that maybe it would be best to place the two fans at 10 and 2 on the clock, rather then the current norm of placing them at 9 and 3. This will mean the two fans work together, and not against each other. Air flow will be more predictable, whilst also giving more coverage then a single fan set up.

Expect more in this space as my noggin comes up with it -- Most of it is conjecture and opinion but I will try and back it up with relevant fact where I can. I might even try and structure some of the ramblings a bit better.