Good question. I was a bit jealous to read that the Brompton's can fit into a standard IKEA garment bag of some sort, and slide cleanly under a bed, or into a trunk. Our bikes are not as compact when folded. There are some 20" folding bike bags on Amazon, but I don't think they'd necessarily be big enough. Qualisports
makes one for the Volador, but the Dolphin is approx 35"x 28"x 17" when folded without racks. A tailor might be able to custom build something out of canvas and cargo webbing (Sunbrella online $25/yard), or if you're handy, you could make one easily. I'd also recommend a $10 rear derailleur guard if you were planning to routinely fold and pack on its side. The rear derailleur hanger is made of a very soft alloy and bends really easily.