A lot of people just didnt worry about it, they just let their 10mm axles sit in the top of that oversized hole.
Otherwise you looking for Some 14mm flatsided axles. From memory I only recall Peregrine doing some of those and they had issues with the wheel nuts stripping coz there wasn't enough thread to grab onto.
One other solution would be to have a couple of top hat axle converters and fit them from the outside once your 10mm axle was in place. That would cure it completely, and hold the wheel centrally in that hole.