The following link references a study of oblique inlet shape for heat exchangers that might be a good starting point for radiator and oil cooler duct construction.  The coordinates below can be scaled for the core size to be used.

http://www.rotaryeng.net/Oblique-flow-diffusers.pdf

X       Z

0       1

0.1   0.89

0.2   0.76

0.3   0.63

0.4   0.51

0.5   0.39

0.6   0.29

0.7   0.2

0.8   0.12

0.9   0.06

1       0