I purchased a Noctua A4x20 (40x40x20 mm) fan after my stock V2 fan started making all kinds of noise and would screech to a halt during prints.
I printed and installed the Monoprice MP Select Mini V2 40mm Fan Shroud by keco185 (https://www.thingiverse.com/thing:2448223). I really liked the mounting design that indexed around the top of the hotend. However, even though the Noctua fan is super quiet and advertises ~5.5 cfm it just didn't feel like it was moving enough air and the backflow was noticeable.
I next to a look at conklech's MP Select Mini V2 40mm Fan Duct design (https://www.thingiverse.com/thing:2301517). This design has a much smoother and more direct path for part cooling. His design for the retaining spring tabs was also very good. Unfortunately only having two holes to mount a fan was a drawback to this design.
I decided In order to increase the throughput and reduce backflow I would re-mix the keco185 design with the conklech design using those things I like about both and also added 2 additional holes for mounting. I use zip ties to mount my fan as I find that to be the easiest and most efficient way for me. The two holes I added are not full mounting holes but work great with zip ties. I didn't spend a great deal of time on that aspect as this is a functional design for me. This design should work with any 40mm fan but is performing very well with the Noctua A4x20.
Printed great at this resolution. No supports required.