Peter's post has reinforced my opinion that the fan shroud is the important item here. The unit I bought is still available on Ebay:
https://www.ebay.co.uk/itm/253887160435
I wouldn't have achieved a better fit on the radiator if I had supplied the radiator for a custom shroud. It was spot on off the shelf. The 12 rectangles above and below the fans are rubber flaps that allow air through when driving along and shut when stationary.
The disadvantages of this unit are noise and huge current draw. As my experience during a hot UK summer suggests a single 11" fan running out of the pair has been adequate I wonder whether running both at a slower speed would be a better solution, but I don't think I am likely to find time to try that any time soon.
Another question is whether straight bladed or curved bladed fans are the better option. I just found this from an Australian fan manufacturer:
https://daviescraig.com.au/blog/stra...xVBoUKQBREwjEC