Here is pic of the plenum (at this point a piece of 1/4 inch ply.  The
ram air scoop will be 4 inches wide and  two inches high.  

Jerry:

Is the 2" x 4" the opening size?  I think that is more than double the area
that would give the amount of ram air needed for a 2-rotor at, say, 120 kts.
So it will be spilling air and likely cause some amount of unwanted drag.
Ideally, a ram induction scoop is close to the area needed for near top
speed; and have radius on the entrance lip (about that of your index finger)
to minimize entrance loss when you are at low speed.

FWIW, I have no idea it any of this matters for what you're doing.

Al