Cut a piece of (3/4/5mm) sheet ali/steel/tufnol etc. into a rectangular shape that fits nicely/fully between two of the flats of the hex. Drill a hole in the middle and put a bolt through it with a nyloc nut on it, or a normal nut threadlocked. Then just use a normal socket on the nut.
If you're feeling flash, you can cut the sheet into a full hexagon.
