Loctite an old allen key into the hole, then just turn.
If araldite and loctite aren't doing the business, a last ditch attempt is to use solder. Just put the solder in the hole, touch it with a hot iron and hold it there while you push in any screwdriver that fits, remove the iron and the solder will go cold, leaving the imprint of the driver without sticking to it.
Be careful of the finish though, this is tried and tested by me on markers, mobiles, tv chassis's etc...need to have a steady hand.