My waterer is a galvanized hanging one but under it is a kiddle pool. When it is time to clean out the coop I fill the pool up with shovels full of the poopy shavings from under the roosts, add the old nesting material from the boxes and slide the thing out the door to dump. We used the pool for a brooder (with higher sides made of cardboard). Not suggesting that you do the pool idea, but anything could work similarly.
