The only problem I can think of is that a tower ofcrates with pot on top will
be top heavy and could tip over in a storm. You
could wire some bricks or rocks to the crates if that becomes a problem.

I use many many milk crates in my ponds ...Remember they will stack, so I
reccomend you fill the bottom one with rocks...any kind... then stack another
on top then the plant pot on it......For a deeper pot, just the pot on top if
the rocks. I had a problem with anacharis all over the pond bottom, so I
filled a milk crate about 1/4 full turned it upside down, and put a brick on
top last summer. It is now totally full, and growing way out the holes in the
crate.....will make a real good hiding place for fry . I also use milk
crates to pot most of my plants. Jerri

http://www.fringeweb.com/Ponds/JerrisPond