When I was young I really wanted one of these but for some reason it never happened (50 new pence did seem quite an unobtainable sum in those days). In the end we made one out of a strip of hardboard with glued on cardboard sides. The girders were drawn in by hand.
I know, it sounds like the sort of construction materials that Bluebottle from the Goons would come up with. Still, we had many happy river crossing battles with it but sadly no images exist, other than in my mind's eye.