My pond is 4-1/2 feet deep and I use 2 of the plastic milk type crates that
I got at an office depot store for $2.99 each. I have them upside down.
they were fine when the fish were little except that the larger fish now
still try to get thru the holes and get stuck. I even cut some larger
openings in each box and the koi dont seem to always want to use the
"doors". I have seen them get stuck and watched them thrash and lose scales
in the process.

Also, last year when I had to catch several fish to treat for ulcer disease,
the milk crates were a catastrophe. the fish are difficult to get to and
net if they can get under those crates. I finally covered the crates with
garden weed cloth (black) and tied each one with plastic strings from my hay
bales. The colored string quickly turned dark from algae and the boxes are
covered on all sides except the bottom so the fish cannot get inside
anymore. I made sure there was enough room between the stack of crates and
the straight up and down sides of the pond for the fish to have places to
hide and hunker down in winter. seems to work better.

just my 2 cents worth.

wendy in Oregon