I'm with David on grinding the head flat. I was just working on a 3rd Rail steamer and I had to do that to the 3mm socket to get it to grip the head. I also had to take it to the grinder and grind the sides down all around so it dropped into that recess.
Back in the bad old days, there used to be a thin-wall socket set for tight quarters, I don't know why the nutdrivers don't adopt that model. It's not like you need all that metal for a 1/2" in diameter handle nutdriver, how much force are you really going to use? So far I've had to grind down a few of my nutdrivers, I can imagine more will get the treatment when the need arises.