Hiya,
Dick Smith had nothing of any use. I have heard of stick on foil types for the inside of glass - they run across the top of a window ( back window usually i think).
Tried jaycar - they had nothing either at the time but they are good for gadjets... try
http://www.jaycar.co.nz. maybe they have something now.
My solution was rough but worked surprisingly well. The problem I had was trying to remove the base of the existing guard mounted aerial. i couldn't get to it without removing parts of guard so I bought real cheap warehouse aerial and pulled it to bits. The extending bit of the new aerial fits snugly into the base of the old one on the truck and its held tight by a short piece of plastic tube. When i go off road and theres a risk of tree damage I just pull it out.
Cheers,
Pete