A cover is not necessary. I have fermented for months at a time and 'bad' bacteria or yeasts have never taken over. And I ferment my feed right in the building my chickens are in. I don't even wash my buckets I use to ferment the feed in.
Adding yeast to ferment feed is just not necessary.
I believe that when this thread was started it was recommended that the FF be covered with a cloth, but I suppose it is a personal preference.
At the least, it would be MY preference, including starting with a yeast. So...