In the 50's, 60's & 70's a UK company called called Craven made a range of fibreglass top boxes and panniers. They occasionally come up on eBay, but any surviving models are pretty tired by now.
Last year I heard that a company called Craven Equipment had started to re-manufacture some of the original range. I contacted them and ordered their smallest top box, which arrived a few weeks later.
It is nowhere near as spacious as DanB's one, and a good bit dearer (£170) probably due to the way its is made: hand-laid fibreglass, with all the hardware then fitted using rivets or stainless nuts & bolts. It does sit perfectly on the rear rack, and is large enough for waterproofs, lunch(!), any extra tools, and the odd bit of shopping. I had already tried on a much cheaper Givi 'jelly-mould' plastic box which I had for another bike, but it just didn't suit the Enfield.
Contact details are:
Craven Equipment
Broad Street, BUNGAY, Suffolk, NR35 1EE Telephones
(01986) 894 798 or 892 826 Facsimile (01986) 894 799
contact@cravens.org.uk
www.cravens.org.uk