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A cardboard box that fits perfectly on the rear deck.
The box originally contained frozen pork loins that I had delivered to my place of employment. When I saw it’s oblong shape I knew that it would fit perfectly on the Yuba so I brought it home to use for future shopping trips…I do this somewhat often, actually (though not with such appropriately shaped boxes)…I’ll bring a box or crate with me on the Yuba and actually bring it into the store…If what I’m buying doesn’t fit in the box I don’t purchase it. A simple solution to keep purchases down.
Urban Simplicty 
>I like that idea. One issue with Xtracycles and Bakfietsen (and I gueaa a Yuba) is tha you can ge carried away because you know you can get almost anything home. Your system sounds cheaper.(exits muttering about containers)