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:

https://www.amazon.com/Lambro-3005-Titefit-Rectangular-Aluminum/dp/B000KKKGJ0/ref=sr_1_2?crid=35OK4JZ384QAE&keywords=rectangular+dryer+duct&qid=1646768171&s=home-garden&sprefix=rectangular+dryer+duct%2Cgarden%2C111&sr=1-2

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