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There were a few options online, some hard cases that would provide lots of protection, but at a price that I really didn’t want to pay. Further Googling brought me to an alternative: the Vincita B132B “soft transport bag”.
The Brompton diaries a Brompton bike enthusiast from the U.S. who travels with Basil her "wonderful" Brompton M6R bicycle.
"There’s a never-ending and vigorous debate amongst Brompton owners about the best way to travel with our bicycles when packing them is essential. No one method suits all, with minimalists going for slipping their Bs into IKEA’s Dimpa shopping bag or the equivalent, and maximalists going tor super-pricey hard cases — with many methods in-between."