*on the belly,the air would be flowing parallel to the ground.If the belly were sealed above the the bottom of the pan,any opening would be seen as a pool of dead air (upside down) and the air would just skip over it like a flat stone on smooth pond.
*Since a void cannot support flow,the perforated panel may act no better than the void it's covering.It could introduce turbulence.
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*On a streamlined bedcover,the solid surface furnishes a deceleration ramp,to slow the air and build its static pressure so when the flow does detach at the back,it's at a higher static pressure,increasing the base pressure,and reducing pressure drag.
*If you use a perforated skin,it cannot perform the pressure recovery,as all the pressure will be equal to that just behind the cab,where it's the lowest,imparting the lowest pressure to the wake,and maximizing pressure drag.
