I think the two below qualify, but have already been discussed.

The ice aircraft carrier
Strange Story of H.M.S. Habbakuk - The War Illustrated
I think the link above in not the whole story as I believe something like it was actually used for training aircrews on the lakes (can't find a reference right now).

And the Canal Defence Light ...
Canal Defence Light - Wikipedia, the free encyclopedia

And let's not forget all those wonderfull british home guard contraptions.