You say the ducting for the makeup air is 12" diameter beyond the black 'thing' in the window opening? Is it possible to extend the 12" ducting alongside the supply duct, drop it down below the level of the bench and let it flow out from under the bench on all sides? Only one 90 deg bend. No need to direct the makeup air in any particular way as the fan will draw it into the capture hood regardless.
A few years back I did some model tests of various vent system configurations and such a system as above worked fairly well.
PJ
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