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the pickup truck increases drag when the gate is open only because the back of the cab is squared off & results in turbulent airflow. the box w/ the top up has a nice slope that guides the air around the car; a result similar to the pickup truck is unlikely.
further, we need to understand the difference between a spoiler and a wing. a spoiler reduces lift by slowing the speed of the airflow over the car. when the air is significantly faster over the car than under it, the bernoulli principle tells us that pressure will be comparatively higher under the car than above it. the result is lift. a spoilier interrupts that airflow & increases the relative pressure over the car, in turn reducing lift.
a wing, instead, develops its own counter-force by generating the reverse scenario of the car itself: it's designed to accelerate aiflow UNDER the wing to press the car down at speed. so there we have it: a spoiler reduces lift while a wing generates downforce.
all this having been said, fluid dynamics is a funny thing.....the air rarely does what we think it will. my guess is that your setup increases turbulence & drag, but does not appreciably affect lift / downforce at the back of the car. your best bet is either the factory setup OR a larger wing, but not both.
blue2000 is a whiz at this stuff; maybe he'll weigh in here.....
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