That's what I do, plus I block about 1/2 of the air return intake on the main level so the system will be drawing in a higher percentage of warm air from the 2nd level. Putting the fan on continuous run also helps balance the temperature.

Also have the attic checked for adequacy of the insulation and proper ventilation. An attic ventilation fan will help.

Well, than your system which is designed for 8+ vents will now have 1 vent? Seems to me you will have a poorly operating system since the original ducting is not functioning properly. Why not add a window ac unit for upstairs? Or a wall mounted mini split system. Your neighbors will be a nvious of your 2 system setup.

Of the two suggestions so far I like stoveguy's best,,,,,,,,,well actually that's the only one I like. Plugging up air flow doesn't fix anything,it just create's other problems. Add a unit , have it professionally evaluated or both.