Be carful comparing this square 'open-box' to a flat plate.
Speculation...
As a practical matter I would expect some odd/unstable 'spill-over' and vorticity formations/shedding off the up-stream-facing 'box-edges'... which would make for hard-to-predict rumbling and vibration modes... and a tendency to have a pitch-over moment.
IF the 'box-edges' were facing opposite [down-stream] the cavity-edges might generate vorticity roll-up into the low pressure aft-facing cavity increasing relative Cd. This might be a more stable shape.
Of course, anything on a rigid mast mounting and of reasonably small size might survive in open-air flow... at relatively low angles of attack [from perpendicular flow].
Regards, Wil Taylor
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