Thanks for your tips!
I don't have to worry about temps as I am in Montana in a basement. However, these are luckily placed in an area with an extra vent (currently plugged) to the outside if needed.
In line fans ... I grow peppers indoors, hydroponically, and when you get into HPS systems the lighting has similar issues. You need lots of ventilation. After my various years growing indoors, I have learned to get a
grow tent for keeping the temps in the 68 - 82 range year. People either use the duct fans or a squirrel cage fan. The squirrel cage is quieter, similar to the R4. I bet some of the knowledge is transferable. Like you said, you don't want to the duct work slowing down the flow - not only heat, but static.