When I was preparing to build my wedge diffuser, Tracy looked at my mold & thought it looked a little too thick at the back (IIRC, it varied from about 4.5" at the front to a bit under 1/2" at the back). When I tested it using a leaf blower & a piece of thread on a stick to visualize exit flow, virtually all the air was coming out the back 1/4 of the radiator's area. I ended up with the back edge of the diffuser almost touching the core, to get even flow across the full face of the core. It's not a straight wedge; it's almost the inverse of the curve you see in a K&W Streamline Diffuser. I'm nowhere near flying, so I can't tell how effective it will be for actual cooling, but at least exit air flow is fairly even across the face of the core.
Cowl flaps: I remember Bill Eslick having issues with being able to close his cowl flap due to air pressure holding it open, and I think he ended up using the flap actuator linear actuator used for RV's wing flaps. I later saw an idea from another friend's Lyc powered RV8. He put the pivot point for his cowl flap near the middle of the flap, so it acts more like a throttle butterfly. The leading edge of the flap is rounded like a teardrop, and the mating point on the cowl matches it, to smooth flow. Just a thought...
Charlie