Just looked at a mill for sale - I think it was Schaublin, and definitely not cheap. The round column had a square key on it to prevent rotation but then the entire column could be rotated in its base. This requires a good sized well fitting base, but it gives a good solution regardless of the type of key used. The column could even be square above the base. Can't see a good way to apply it to any Unimat I've ever met.
