This is the inline fan I use for mechanical ventilation:
https://www.amazon.com/dp/B07JNGS2V7?ref=ppx_yo2_dt_b_product_details&th=1
Works great! Comes with a digital speed controller with 8 speeds to fine tune ventilation for sauna size and occupancy considerations. At 140cfm max speed....setting 1 is 12.5%=17.5cfm setting 2 is 25%= 35cfm and so on. It is silent through speed settings 1-3 and also comes with a dial variable controller as well.
I used half of this rectangular dryer vent to bring in fresh air above the heater:
And also this backdraft flapper valve in-line with the dryer vent that remains closed during warm-up and then opens once I enter and turn on the fan:
https://www.amazon.com/dp/B08TC1W61T?psc=1&ref=ppx_yo2_dt_b_product_details
TerraBloom ECMF-100, Quiet 4 Inch Inline Duct Fan with Variable Speed Controller – Ventilation Exhaust Fan for Heating Cooling Booster and Grow Tents https://www.amazon.com/dp/B07JNGS2V7/ref=cm_sw_r_apan_glt_i_ZNQ10T3RJWMS3618Y84Y?_encoding=UTF8&psc=1
The in and out ducts are different sizes, I just modled and printed an adapter. It fits the filter snugly. I usually keep it on level 1 or 2 when not in use, 4-5 while printing the fan gets a little too loud at 6-8 for me to tune it out. I've asked several people and they couldn't smell anything in the small office I have it in.
VIVOSUN 4 Inch Air Carbon Filter Smellines Control with Australia Virgin Charcoal for Inline Duct Fan, Grow Tent, Pre-filter Included, Reversible Flange 4" x 14" https://www.amazon.com/dp/B01DXYMBU6/ref=cm_sw_r_apan_glt_i_FZV0VFRSPXXT22F5GSG4?_encoding=UTF8&psc=1VIVOSUN 4 Inch Air Carbon Filter Smellines Control with Australia Virgin Charcoal for Inline Duct Fan, Grow Tent, Pre-filter Included, Reversible Flange 4" x 14" https://www.amazon.com/dp/B01DXYMBU6/ref=cm_sw_r_apan_glt_i_FZV0VFRSPXXT22F5GSG4?_encoding=UTF8&psc=1