Recently, I have searched for backup data for drag forces on mesh and found nothing too helpful for the cases I have had. I have called the manufactures and they essentially told me that it is the EOR's responsibility to figure it out (jeez). That said, Cp = .5 sounds really low. If the mesh is flexible and is able to "dish", I can't see Cp = 1 even with the proposed reduction of closed area. Frankly, I wouldn't consider anything less than 1.0. I did find a masters thesis on the subject, but I didn't trust it and not sure if I could even relocate it.
Also: Even if you can justify a lower drag coefficient, I would check the wind on ice load combination in ASCE 7 (or similar check in whatever code you are using) considering the mesh as a solid panel, if of course this mobile structure will see ice. I am sure that would govern in most cases.
"It is imperative Cunth doesn't get his hands on those codes."