See http://bestbackrests.com for all the details.

Isuzu I think, I read about it on an American Burgman site as a cheap mod.
G.

I spent a frustrating couple of hours trawling through a scrap yard last year, couldn't find any Isuzu's but did get a leather BMW 5 series headrest that is the right size to swap the OEM Suzuki B650 backrest.

Only issue is the design of the BMW was more padded and sausage-shaped rather than the simple squared Isuzu design. It certainly provided great lumbar support, but it actually pushed you further forward as it sat lower and ahead of the Isuzu version.

I found it very good for in-town work when I was more upright and alert (Britain needs lerts), but not comfortable enough for long-distance work.