[aerohead]

Here are some structures tested earlier on:







In a zero wind condition you can get full attachment with a radius equal to around 4-5% of the vehicle width as mentioned by members.
In wind conditions the bulbous nose has an advantage.These cod's head noses while best,are most difficult to fabricate.Top and sides need softening.
In times past,tow vehicles would carry a roof mounted inverted boat to aid flow onto a caravan trailer of matching height.
The prow of a boat can have very good air penetration capability as you might imagine.And even with added frontal area could reduce overall drag if carefully configured over a sharp-edged camper shell.