Hi Steve,
Referring to the top view drawing, my guess is the majority of the air would favor the top half of the core, starving the other half of air. I think I would try to duplicate the long curved duct wall on both sides, forming a trumpet shape nearest the core. On the side-view drawing, my guess is the majority of the cooling air would pile up on the back/downwind side. I would squeeze it down to almost nothing at the backside. Then take some manometer readings and adjust as appropriate.
Be sure you have free space behind the radiator for the air to exit the core, and adequate cowl opening(s). I had to add a cowl flap to my Lancair, but it really helps during hot weather climb-outs. When up into cooler air I usually close the cowl flap for reduced cooling drag.
Mark S.
Lancair ES - PP 20B - 258 hrs