I used the same principle as Jason and Mike but not quite as pretty
http://www.flickr.com/photos/3310545...in/photostream
Although once it's painted and mounted you can't tell. I used some big washers for the bolts into the tub. I think the point about being able to remove the spare wheel without access to the underside of the tub is important.
And I didn't fit it for the IVA (re-test) as it failed the radius test first time round, weirdly, as the wheels themselves passed ok ??!!
Tim