I guess I'd figure something out with latches.
If I were going to do a plastic crate again I'd use a soda crate and not a milk crate, it would fit under the seat better.
There are folding baskets.
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Genesis 49:16-17
"Well, well!" said Holmes, impatiently. "A good cyclist does not need a high road. The moor is intersected with paths and the moon is at the full."