The geometry of your model is quite messy, basically because you’ve been manually trying to make a 3d curve which can be quite difficult.
This messiness makes it very difficult to work with to get simple clean thickness, even with fredo’s Joint Pushpull.
Personally I would redraw it to make it cleaner and easier to work with. You could also improve the airflow from what you have there.
Here is a quick one using a bezier curve, curviloft and joint pushpull. You see it creates a nice smooth solid with thickened walls. Making one half allows you to just flip it and join them together. By making the mouth wider than half you can slice off part of the duct where they join to get a nice V throat. All three extensions will work with 2017 make.