I decided to do some testing to see if it was worthwile cutting a hole in the bonnet to allow the hot air inside the bonnet to escape.
This is for those interested in a bit of scientific evidence, not just opinions.
Test method:
I have a digital manometer, with with dual inputs, one input I put on the outside of the bonnet, and the other one on the inside of the bonnet, just behind the radiator.
This i quite a precise little instrument, measuring fr0m 0-5psi with an accuracy of 0.001 psi. It is meant for air conditioning balancing in building, so needs to be able to show small differences.
The next thing I did was put some wool on some tape all over the bonnet. I just wanted to see where the airflow was going.
Then I did a run down the highway, from 0 - 100kph, and did if in both directions to see if there was any headwinds or tailwind effects ( there was a little bit of a breeze, that all)
I did two runs, a few days apart. The first time I did it, I'd set the pressure gauge to the wrong setting, and it didn't read anything. The second time I got it right.
The Results
The pressure inside the bonnet was always lower or equal to the pressure on the outside. The maximum difference was 0.060 PSi according to the gauge.
The wool tufts showed pretty clearly that close to the windscreen, the wool went inside the engine bay, through the gap between the bonnet and the cowl.
At speed, the wool was blowing backwards, away from the windscreen at theback of the bonnet, and blowing towards the windscreen in the front of the bonnet
I did a bit of a video:
My conclusions.
The fact that the pressure inside the engine bay than outside the bonnet is to be expected. The radiator etc all added to the wind resistance. I have an electric fan, which did not come on for the test, and even when I switched it on manually, I could not see much of a difference on the gauge. When the truck is not moving, the gauge does not read any difference with it on or off.
Putting a hole closed to the windscreen would make things a lot worse, as you would be getting more positive pressure inside the engine bay, which is the opposite of what you want if you want as much airflow through the radiator. I now have a rubber seal on the back of my bonnet to stop the air getting into the bonnet from the high pressure zone in front of the windscreen
A hole just behind the radiator like on the UGN trucks, does not seem to be worthwhile with my style bonnet and grill, as there is more pressure outside where they cut the hole than inside the engine bay. I think the grab handle will have a positive effect, but here are much better aerodynamic ways to get it so suck air out the bonnet.
In the end, the goal is to reduce the pressure inside the engine bay are much as possible, so that there is the biggest differential between the front of the radiator/ grill and the inside of the engine bay, so as to maximise the airflow through the radiator. Anything that increases the pressure inside the engine bay is opposite is what you want, and I think that cutting a hole in the bonnet will actually increase the pressure inside the engine bay, not decrease it.
I theory you could put the reverse scoop on, using the aerodynamics to create the low pressure needed to pull the hot air out the engine bay, but the airflow says it should be at an angle to work, and it would look pretty ugly though, I am not sure it is worth it.
I have an electric fan, so maybe having the big steel fan running all the time may shift more air, but switching on both pusher and puller fans made no difference on the gauge at all, either when driving or stationary.
This is my civil engineering view, if there are some aeronautical engineers around who can point out what's wrong with my conclusions, then feel free. IF you have a hole in your bonnet, and believe it is working, then do a simple test, just stick some wool in front of the hole, if the wool blows out, then there is air coming out the hole and the vent is working, but if the wool gets blown into the hole, the you have made things worse and should plug the hole to maximise airflow through the radiator.