All you need is 3 pieces from you ratchet kit:
A cardan (most kits have write "universal joint");
And finally the box key you need.
To assemble connect the box key to the cardan and the cardan to the extension.
Now bend the cardan 90 degrees while putting one of the talls in the cardan in one of the cardan's reentrance, like in the picture and you are done!