I fill mine up about a third from bottom to give a little head room for if the ferment gets violent. For 2 or 3 week ferments I just use water, for my cider and mead that will sit for several months I add a alcohol water mix.

Some people have added mouthwash to cover the smell, but what ever you use make sure it is safe to drink as it may get sucked back in the bucket, especially if you were to lean on it.

I actually just got up and looked, I have 4 double bubbles and five 3-piece airlocks going - all making a nice sound, kinda Pavlovian.

Oh yeah for my finished washes that I rack to clear I just use some cheap 15" kitchen towels to cover the five gallon buckets and lay the lid on top, I usually get to them in 2 weeks or so with no problems.