I have one that came with my car. Previoys owner had it fitted. (Richard Thorne I think fitted). It came off when I had the rollbar fitted as brackets were in the way. I have not fiddled with it to refit as not easy due to rollbar(Librands). It did reduce wind a bit when tried but not hugely succesful. The airflow seemed to come down in front of it after looping over from the windscreen.
Maybe a wrapover type on the rollbar would help. A bit like a smal "shelf" to catch the downdraught and a rear drop down behind the Rollbar, forming a type of targa effect.
Got me thinking now.
