why would anyone need more holes on the roof anyway? they already let enough water in :)
other options for antenna mounts include ones to go on the spare wheel bracket, and there is also a light bar or two with an aerial mount attatched. Failing that, get a mirror mount, and put it around a light bar, or use an old roof rack.
on an astra van i had years ago, i bolted a 4mm thick steel plate across the roof bars, secured with u bolts, and drove around with a 7ft aerial on the roof, as good as drilling into the roof, but easily reomvable without leaving a nasty hole behind.