Yep, I just drilled a normal hole that ended up big enough for the square to fit in. It's best to use a drill bit called a "hole cutter". I got mine from the local tool supplier. They're a cone shape drill bit with lots of steps to give different sized holes. You start by drilling a fairly small hole with a normal drill for a guide, then place the hole cutter in the guide hole drilling your way up through the sizes until you get to the where you need. It's much safer and you've more control than using different size drill bits. It's also worth covering the roof in masking tape as the swarf scratches the paintwork real easy.
If you haven't got one already, make sure you've got the proper nut that goes on the antenna. Its got a spiked washer built into it that grips the underside of the roof and leaves room for the cable to go through.
Good luck.
