As codenamezero indicated, the point of a belly pan on the fit (or mine at least) is to reduce turbulence. I don't have a link handy but I saw some figures onthe biggest offenders of turbulence under a car which prompted me to build my pan. Those major drag offenders include the front wheels, the area under the engine bay (which is why most new cars have a least a partial plastic under tray) and the rear wheels.

 

I have the BeatRush front panel and I have noticed that it actually stabilized my front end during freeway driving.

As for the rear wheels, have you looked the rear wheel arches? There is no paneling/rubber or plastic wheel arches covering at all. If you plan to make a rear panel to match the belly pan, the rear arches must somehow be covered to prevent it from acting like a parachute.

 

I think if anyone wants to make a drag reduction belly pan, then the design would be just make it flat straight starting from the gas tank. we don't know how the EV Fit rear diffuser look like yet, but i would still assume it should curve up a little, but not as steep as the racing rear diffusers... I think as long as we don't speed up the air flow under the car, then it won't create as much downforce.

Making a flat out belly pan is probably the easiest way to do it, but the better design would be sorta making it like the EV Fit one, with vertical grooves from front to the back which helps directs the flow and reduce turbulence.