Personally, I'd say don't bother. I installed one on my last Tornado, an 8.5 which had a really tall A frame. I did a little training in conjunction with the local RNLI ALB, and it could never see me on radar, no matter what the weather!
I then fitted an Echomax MIDI (hanging down from the A frame!) and my boat was visible on radar.
So if you really want a visiball, I've got one lying in the loft that you can have for nothing!!!