This is how I plumbed the exhaust from my diesel heater through a 3" flue thimble using a stainless steel exterior port. I have ran this for nearly 7 hours on high with no heat transfer to the wall materials. This heater is able to maintain a comfortable temperature in my 2 car insulated garage, just don't expect it to rapidly bring a frozen garage up to 70 degrees quickly!

Links:

SILVEL Diesel Heater
https://a.co/d/8T8m095

Exhaust Port
https://a.co/d/gN9twTZ

30 Amp Power Supply
https://a.co/d/653lonL

Ceramic Fiber Insulation
https://a.co/d/bvtpXgS

3" Wall Thimble
https://a.co/d/eOJz2j1

5" Flue Cap
https://a.co/d/10SlzPk