I seem to remember in the dusty recesses of my mind, reading in a manual somewhere about covering poultices and plasters to help retain the moisture. I am trying to go back through my widely separated resource material to find the section. It seems to me that oiled silk may work well as it would retard evaporation but be flexible enough to move with the patient.
your obedient servant,
Surgeon, 2nd Brigade
"not really a surgeon, but I did stay at a Holiday Inn Express"