Here is some data from Ahmed and associates which includes plan taper at the rear as well as some diffuser.
Every 'best' angle seems to be contextual and we really have to stay on top of the conditions under which results were obtained.
*with only roof angle we get 9-degrees
*with 10-degrees simple plan-taper,and 10-degrees of diffuser our drag is lower with 25-degrees of simple roof angle.
And Ahmed et als. research gives only 9-configurations out of hundreds we might consider
With Hucho,we see some numbers with frontal softening,edge softening,and 5-degrees of simple contraction for both sides and top,with zero diffuser.
