I normally stack 3 or 4 crates...then they look exactly like a rock on the bottom...if you put them in rocky areas, no one will ever know they are there except you.
I try to get 10 together in a "stump field" or mix 3-4 in with other cover to provide Crappie with all options they need.
My luck was phenomenal...my best friend moved into a new place that had an old barn that had over 1500 milk crates....he gave them all to me to get rid of them...they are all in our 2500 acre lake...and you can't "see" a single one! Lol