New zone with one vent

Hello everyone, I have central HVAC system. Bosch Gas furnace, 120,000 BTU, forced air, up-flow (Model: BGH96M120D5) with currently 2 zones ( 1st floor w 2 dampers and 2nd floor w 1 damper). Basement has 2 small vents part of zone1 which not enough to heat the basement. Basement has 1 return. I want to add a 3rd zone for my finished basement (1200 sqft) and close the 2 small exiting vents. I'll use it mainly for heating not AC. I can't run multiple vents in the basement.. I can run ONLY 1 vent and 1 return. Control panel is Honeywell HZ322 and the 3rd zone is free. There is bypass installed connected to the right side of the humidifier ( you should see it in the second picture) . I have few questions. 1) Is that possible? or it's too much force for 1 vent and it will be short run too 2) If it's doable with one vent, how big is the vent should be and how big is the return? 3) does it matter if I cut it circle or rectangular? Thanks in advance! Image may be NSFW.
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Clik here to view. Image may be NSFW.
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