I never did get a picture of the finished product. However you can see the vertical lip that was leading the opening. That flap was used to rivet the upper air guide in the final version. Also, a flap such as this is better than a smooth bulge from what I recall.
The image below, the last image has the additional air guide I have been mentioning, which is crudely drawn. Note the changes from the middle picture and the lower. The upper air guide protruded above the hood roughly 3/4". It also reduced the overall vent area, which increases air speed (The closer velocity from different air streams that merge, the less turbulence).
Food for thought. An opening on a flat surface, with high velocity airflow; will create a reduction in down force. Hoods of cars experience higher velocity airflow and a poorly designed vent will cause air to either go into the vent, or ruin good airflow characteristics that already exist. I've experienced this first hand with the Rx7. The first two open vents (picture above: no guide, and lower guide) caused the front of the car to feel very light (vague feeling) compared to a normal hood. The addition of the upper and lower air guide provided a smoother transition for the two air streams to merge, which increased down force (reduced lift from turbulence). However, the vent I was using is considerably larger than what most can even do on an Alltrac, so this won't be as much of an issue. The largest concern is ensuring air flows out of the vent, and not into it. Should air flow into it, you are increasing pressure behind the radiator which will reduce it's efficiency (counter productive).
Decided to make a quick paint sketch...
Crude paint drawings of how air flows over a hood with a duct or vent of various designs. A and B are the same vent, but different scenarios of how air may interact with the vent.
A: This is counterproductive when moving as air flows into the vent. This increases air pressure behind the radiator, and reduces overall cooling ability. This is very detrimental for cooling, and will cause a lower pressure zone above the vent (increased lift). Vent design, and angle of the hood will have a great impact upon this.
B: Small amount of air may flow out while moving. Chances of air flowing outwards at speed greatly depends upon under hood pressure forcing its way up. Since under hood air is slow moving, this would cause drag/turbulence in all likelihood.
C: Forward leading edge causes eddy currents in the airflow. High pressure before, and low afterwards. Low pressure will assist/encourage air to flow outwards. The lower duct which guides air out increases the speed of air exiting to an extent. Airflow exiting will still be significantly lower compared to velocity of the main airstream.
D: Upper and lower air guides increase airspeed significantly. Vent area is reduced, but efficiency increases drastically. Air streams are more closely matched with velocities and minimal drag occurs. This is an ideal vent when ducted to to radiator directly. With proper ducting before and after the radiator, cooling efficiency should increase, and front end lift minimized. Should proper ducting be used, it should be possible to increase down force.