I place blocks of fondant on the queen excluder directly over the cluster (the excluder stops it from oozing down between the frames).  Even in what one would think is a moist environment, it is essential that the top of the block is covered with plastic to stop it from drying out; I do this by cutting the block (which is packed in plastic) and then placing the cut side down.

http://www.stratfordbeekeepers.org.uk/PENotes/Fondant.htm
Note that these pictures date back to when my bees were more yellow - I now only feed half a 12.5 kg block per hive.

Best wishes

Peter 
52°14'44.44"N, 1°50'35"W

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