Use your bench grinder to grind away enough of the end of the 10-mm socket to leave something that resembles a spanner. Grind it so that the thickest part of the wall of the socket forms the spanner, i.e., the part of the socket with the flats. These have a round blade that is coated with tungsten carbide that will cut hardened steel. Saw two parallel slots into the end of the socket in a way that will form the blade of the spanner. Cut the excess away by sawing from each slot to the outside of the socket.
Masochists can use a file to remove enough metal from the end of the socket to form the spanner.
Since chrome is harder than files, it will be necessary to break through the plating with something like a sharpening stone before commencing the attack with a file. A completed tool is shown in Figure 3. It's the correct OD to fit inside the inner return spring. There is a lot of room for slop here. Squaring up the spanner blade.