You have to watch the air pressure areas. below the windscreen is low pressure so a bonnet vent at the back of the bonnet will probably suck air out of it rather than into it.
The standard way of doing things is to put the intake as close to the front of the car.
I have a reverse vent on the top of the bonnet to get the hot air out, as mine won't go under the car thanks to the sump guard.
Cheers
D
Could you do what the mini's do and raise the rear of the bonnet an inch of two. or just put 3 or 4 holes along the back??