The outlet is just any opening behind the engine where air can go through.

Blow in a tube. Where you blow in is the inlet and the other end is the outlet. In this case you want the hole in back to be bigger than the hole in front.

Or... if there is more than one hole in front or back then the sum of the holes in the back should be bigger than the sum of the holes in the back (total area).

What you're thinking is fine. A lot of people used to use the black bottoms from the old style 2-liter soda bottle for cowls.