That should work great for slow moving on trails, but I'd be careful to watch your temps when you're driving uphill at highway speeds. The area in front of the windshield is a high pressure area and opening the back of the hood like that means that air will probably be flowing into the engine compartment through that opening, not out of it. Having higher pressure under the hood could potentially reduce the airflow through the radiator, reducing your cooling.

Kudos to you for trying it out, I don't know that anyone else has done anything like that before. Obviously it's helping to vent underhood heat at slow speeds - I'd love to see some airflow testing to see what it does at higher speeds also. You could tape some pieces of yarn to the rear underside of the hood to see what direction and strength the airflow is at highway speeds.