I have just noticed that 7mm scale GC Wagon Tarpaulins (or wagon sheets as they are sometimes known) are available from C&L Finescale. Also available are CLC and a number of other companies including even the Cleobury Mortimer and Ditton Priors Light!
Wagon sheets were extremely commonplace in the GC era when many loads we would now think needed a covered van were carried in opens. (Amazingly even loads such as flour were sometimes transported in this way!) The sheets were also used for temporary repairs to leaking vans and also on cattle wagons, especially in cold weather or when in use for carrying horses. You really cannot have too many, though most of us are reluctant to cover up our careful detailing and lettering under an anonymous sheet!