Aha,
I've seen those used on motorbikes, with the special tool. Are these any good?
http://www.tridon.com.au/Products/Pr...18659&P=387314
http://www.tridon.com.au/Products/Pr...18662&P=387361
Another thought, you might be able to start with a standard 1/2" drive socket the correct diameter, and with a bit of careful work with an angle grinder, make one.
Cheers