Come off the registers and it'll be alot easier and cheaper. If not , yellow is very correct, make a perforated outline of how you wanna cut the whole with the drill. Also you'll need some aluminum duct work tape to get a good seal on it, or you'll be losing pressure and energy efficiency. Also cut on the inside of your line when you mark it off with a sharpie, it's always easier to flare the end of the duct fan with tin snips for a tight fit than cut a whole bigger than a blowed out pornstars snatch. You're headed in the right direction! Thats alot easier with it run up in the attic like that, that's why I did mine off the registers cause i'd have to crawl my fat ass under the house, and That's not gonna happen. I killed over 30 Black Widows cleaning out the garage setting mine up, no fucking way i'm gonna get bit under the house!