I intend to cut a new outlet on the side of the main
duct, fit the proper fittings ( two 90 degree bends shaped into an _|`` form,
which should redirect the airflow

You should reconsider this plan. Any 90 degree bend in ducting rduces airflow
considerably. There are charts trhat describe these losses that some our NG
HVAC experts can comment on better than I can. A pair of 45 dgree fittings may
accomplish what you need with more effficiency. Talk to a sheet metal shop tech
about a 45 dgree outlet on a new section of ducting. Price ought to be nominal
and could make the job easier. Good luck.

Joe