Thought somebody might be interested in the dart top box rack just appeared on eBay.
I have one of these and they are an excellent period accessory, fitting to the rear frame without the need for modifying bodywork so 100% reversible.
I used to hate putting luggage on my dart for fear of damaging the bodywork no such worries now. And you can do two up riding with a top box now too.
Not cheap but we’ll worth it in my experience
https://ebay.us/Hv7yYF