Adding return air?

[cjt]

I am of the impression (based on living there and some expert opinions) that I am lacking cold air returns in the second level of my 2-storey house. All that I have currently is one cold air return at ceiling level that has vent openings on either side of the wall through which it runs.

I have access, thanks to a central staircase, to the wallspace that runs from the basement to the second-storey. I am wondering what I should consider in adding return in at least one additional location on the second floor. There is a return line that runs to the first floor directly above a bedroom that has no return on the second floor. I can access the joist space above where the return duct currently ends all the way up to the second floor.

Would it be enough to cut a hole in the bottom plate of the second floor stud wall, add a vent and sheet metal between joists to continue the duct to the second floor? Anything I should be worried about? I would only be able to add the duct at floor level without drywall damage and repair. Would it be better to run a round pipe/duct to the new return and tie it into the existing ducts?

I got a quote for $400-600 to have this done, but I hesitate to agree to that since it appears relatively straight forward.

Any input would be greatly appreciated.