Now see, this image shows a very simple hood. Bot notice, it has a double back wall, that's how it forces fumes to take a certain path. This model is molded, not fabricated. I think you will need the help of someone who really understands aerodynamics. And I don't know anyone on these boards who does. Still, you're already doing the smoke test, so you may be getting there.
A little while ago, I was doing some ignitions, that requires me to add conc. sulfuric and conc. nitric to an organic sample and heat until fumes cease. I kept the sash all the way down, for everyone else's safety. And fumes came right to the sash. I asked "Shouldn't this hood be doing a better job?" and some one raised it to above midpoint, where it said it should be, and then the hood sucked the fumes right ou the back. So, the aerodynamics of hoods isn't trivial.