If the shaker scoop isn't make a reasonable seal against the hood, try raising it up a bit until it is.
The scoop height is adjustable. There is a tip here (Tip #1),
http://www.mach1registry.com/shaker/
There is also a video to explain how (about 3 min into it, skipping the stuff about the over spray paint),
https://www.youtube.com/watch?v=ymAUQgpsJF0