You surely want perfect control and adjustability. To achieve this, there is no SHORTCUT! The ganged twin flaps must be identical in every sense. Both must close at the same angle and be so tight, that direct sunlight will not pass!
Only a lathe will deliver the precision needed. The tubes must be lathed inside first, and then the flaps can be made accordingly… 3-4mm thick brass makes wonderful flaps.
Without stress the quality will appear! Good wishes!