6 inch inline fan with a dimmer switch hooked to your hood should be quiet at medium and keep your light cool to the touch i run 600watt hps and have 4 inch vortex inline i wish i got the 6 inch so i could run it at a lower speed gonna be more then $100 most likely but unless your somewhere thats cold all the time 600watt in a small space gets hot fast w/o good ventilation at least for me
i have 2.5ft deep 7ft wide 8ft tall closet that iv been able to get to only 4~5 degrees hotter right under the 600hps then the room its pulling air from
the hood is vented with 4inch inline then i have a small ocliating fan pointed under the light and over the tops of the plants and then my intake fan at the bottom cornner pointing between the pots bringing cold air up mixing into the ocliating fans path and being vented out im probably doing it wrong but its working atm lol gl