I'd go with what Peter suggested using a crescent wrench, but instead hold it perpendicular to the panel, so the (usually rounded) jaw tips only engage two of the flats on the nut. If there's enough room, you can lay the wrench flat on the panel, but usually there's not, so attacking from above helps in tight spaces. Also, use the adjusting screw to keep the squeeze tight on the flats while you go, since the jaws tend to have some free play that can be taken up - practice makes perfect.
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I keep an assortment of those cheap crappy open-ended "wrenches" (if you can call them that) that often come with assemble-it-yourself furniture and such - the kind punched out of thin sheet steel. They're flimsy but handy sometimes to fit in tight spots, being easy to bend to new shapes, or file the jaw opening bigger, and so on. Also, because they're so thin they can get under and between things where a real one just can't go. They can't torque like a real tool, but often it's enough, and not a big deal if you ruin it.
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Ed
vk2bea
Sam Reaves