design in progress... a little help needed

[dokken6996]

what i have here is a design i was thinking about making because i want to start a setup in my attic but dont have a lot of space here are a few crappy pics i drew in paint it looks a lot better on paper haha i think i pretty much have sumthing that will work but as im looking at my pics i see a problem with my ventilation im planning on using an air cooled hood on account of the small space and the heat but i need to figure a way to draw fresh air into the hood and the box from the one single duct. in the pic u can see i wanted to use a y connecter but i relized i need a fan on all the time on the duct that goes to the light, right??? well i cant have a blower fan on all the time, i mean i can but do u think it would cause a problem being on 18 hours of the day?? and would it be ok to have that much constant airflow going through the box i heard u dont want constant fresh air but intervals, but in my mind that doesnt quite make sense if anyone has anything they would like to add or make suggestions about please feel free i wanna try to get as many kinks out as possible before i actually make this