David Elfstrom on Twitter: "Important! Get the most out of your #boxfanfilter with a shroud on the outlet. Improves inlet flow & reduces power and noise. I've found optimum size for two
Permatron PreVent Fan Filter Shroud FanGuard - Washable – Air Filter Hub LLC
Mishimoto Aluminum Radiator Fan Shroud Kit 2002-2007 WRX/STI - FastWRX.com
Amazon.com: Tinker3D Antminer S19 T19 S17 T17 Double Fan Shroud 6 Inch Outlet to 2X 4 Inch Round ASIC Miner Air Cooling Duct 2X 120mm USA Made (1) : Electronics