I am installing a new range hood for our kitchen. The duct exhaust at the top is 8" circular, but our old vent to outside is 10"x3.5" rectangular. I see what we could usually use a register boot but there is not enough space between the exhaust and the vent. The wall vent is just 1-2inches above the exhaust vent flange.
Is there a different HVAC part I could use? Could I build my own from metal or ductboard?