I unintentionally tried something very similar to the "piece of perspex on the roll bar" still of wind deflector when I was moving a load of parts in my Seven a while ago. I had to take a spare windscreen with me so I taped it to the FIA roll bar, just behind the seats, in the same place as the piece of perspex mentioned previously.
It didn't work. The buffeting was worse, in particular you end up with a lot more wind whipping round your neck. I think the JJ solution looks to be a lot better.
Mike (in sunny Wellington, NZ waiting to spot my first Caterham - does a Lotus 7 Mk.IV count ?)