Looks like your venting through an exterior wall. That makes it easy.
I can't tell from your photo, but if the rain-cover-box thing over your blower is more or less sealed, it's not going to be able to move much air at all. A couple drilled holes isn't going to cut it. The intake volume = the exhaust volume...in other words, blocking the exhaust is the same as blocking the intake.
Open Prop fans move a lot of air, while ducting creates a lot of drag. With 24" of 8 inch ducting, you're no longer pulling 495 CFM.
I've had in-line duct fans rated at 3000CFM pushing through several feet of 12" diameter pipe that pulled less air than a 1000 CFM attic fan.
If it were me, I'd be installing a fan like this in the wall, on a rheostat, with hood built from hardi-backer and 1x2's.
0-1650 CFM, $100.
Not cute. Not very quiet.
My 2¢.
Edit: I am not a ventilation expert, just speaking on what has worked for me, from personal experience. I hope you find a solution that provides adequate airflow.
Stay safe!