This came about as i had just installed new noctua case fans in my pc. They have such an improved flow rate that they drew in the front filter and caused a buffeting noise. (yes the filter was clean)
So I then mounted the fans to the inside of case, problem solved, except there are now many places for air to be drawn in unfiltered.
Hence the shroud was necessary to duct air predominantly from the filter.
Its far from perfect as far as gaps are concerned but does the job much better than nothing, and the PC is whisper quiet now.
Just use your the original fan mounting screws to install.
The author marked this model as their own original creation.