28 X 28 X 1 Round J Channel Louvered Aluminum Exhaust Vent 

https://onyxxteriors.com/info/explore/28-X-28-X-1-Round-J-Channel-Louvered-Aluminum-Exhaust-Vent.php

Round gable vents for vinyl siding, aluminum and steel siding being pre J'ed out makes the install so much easier and faster. Flex J channel is not as easy to install as it sounds if you are trying to keep the ripples out of it.