[aerohead]

The OEM exterior surfaces on ZEVO would constitute a 'Class-A' finish, rendering a 'wetted surface-friction drag coefficient ( Cf ) of 0.003-pound/square-foot of area.
It's not beyond the realm of belief that, the increased surface roughness of the mods, and especially the plywood tail surfaces themselves, could allow an 'unnatural' turbulent boundary-layer thickening towards the tail's trailing edges, which would reduce momentum transfer from the inviscid flow, 'down' to the tail's surface; compromising the pressure recovery, compared to what might exist, should the tail comprise a class-A finish.
Which is a long way of saying that, you may have already reached your 'target' drag reduction as it stands, however we can't 'see it' because of 'smoothness' issues.
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My 'gut' feeling is, that, if you had a trailer light kit on the tail, and went ahead and closed off the existing fenestation which allows observers behind ZEVO to 'see' the OEM taillight/turn signal units underneath the 'floor,' closing off that entire area with a second 'floor', would get you 'beyond' your target.
The lack of the rear-most section of belly pan, plus the lack of diffuser, on a passenger car, would be costing you something on the order of Cd 0.035.
With that, and 'smoothness', you might 'hit one out of the park' drag-wise.'