There was a fairly extensive thread on the Canard list a couple of years ago about exits.  The subject of reversed NACA inlets came up and some pretty authoritative people said they don't work backwards.  The consensus at the end was that the best exit is probably a rectangular, gentle ramp with a sharp edge and the ramp extending into the cowl a ways past the edge.
Then the conversation shifted to aileron gap seals which I have found to be very effective on my Velocity
As best I recall ... Jim S.

JJ JOHNSON wrote:

I have never seen a NACA scoop used to vent air FROM the aircraft BACK into airstreams.I have reviewed all the NACA papers on scoops and have never seen this effect described.  Please point us to any reference on the subject. Respectfully,Eric