I agree with using the old mailbox. My yard is about 400+ deep and it would be a long walk back to the house if I forgot the gloves and pruning shears. Keeping items like that in an old mailbox makes it more convenient.
Had trouble keeping pots from blowing away, falling off shelves, etc. So here are my two ideas:
Small pots stacked and stored inside an old milk crate.
Larger pots stacked and threaded onto either a garden stake or a piece of rebar. Stick the stake into the ground or, as in the photo, into a large pot of soil.