Here's some inspiration from various UK outlets.
Simple option seems to be to tie them on the roof rack as already suggested.
Maybe weld something onto a custom mount on the back also?
See the Defender? I like the idea of a 'custom' window frame for the FL (they aren't made). To do this we'd have to get a large group buy to make it cost effective and someone to kindly manufacture them for us to fit the shape of the Freelander! Also you'd need to take your side panel trim out to fit to drill the bodywork. Bettter Pic of Defender window grill linked:
http://www.extreme4x4.co.uk/acatalog/ssteelgrills5L.jpg