Duct Connector Flange 4 inch, Metal Straight Pipe Flange for AC Exhaust Ventilation & Heating Systems (95mm, Silver)

Not claiming it's the best way because it sure looks janky, but I found that mounting the fan directly to the back wall really amplified the vibration from the fan. To quiet it down I made a little stand out of pressure treated 2x4 and set it away from the sauna structure an inch or so, and connected ducting to a surface flange ( https://www.amazon.com/dp/B0DYYSKDCS "Duct Connector Flange 4 inch") that I screwed to the wall over the 4" hole. I mounted the fan with flow facing down, and put a soffit vent cover on the outlet to keep pests out: https://www.amazon.com/dp/B0FQW86D18 "4 Inch Soffit Vent Cover, Great for Bathroom or Dryer Vents"