In article .com,
wrote:
Robert - That's it! Great solution!

Almost painfully obvious, once it's been pointed out, isn't it? grin

I can put an external plank on each
of the shorter sides to hold them out flat, ye-oldy-castle-door style!

One caution on that, the supports for the castle-door 'bar' have to be
on the _long_ pieces, in order for it to fold up 'flat'.

Or, depending on how much time you're willing to allow for install/remove,
you could run carriage bolts (head on the _inside_ of the container) out
through the reinforcing bars.