Find a 13/16" spark plug socket - they should fit over the 21mm nut and sit on it snugly. Spark plug sockets usually have a hex cut-out on the head of the socket so you can tighten it with a spammer while you hold onto the allen key.
Be gentle and make sure the allen key is held down - too much twisting and you can break that part.
Also, deep O-ring spanners do the job pretty well too, if you can find one in the 21mm size. My new mounts (HD ones from GAP) came with smaller-sized nuts so my smaller cheapo L-bent spanners from SuperCheap could be used. They come in handy that's for sure.
On a side note, GAP sells a tool for that if you want to actually torque it down properly -
http://www.germanautoparts.com/Tools...spension+tools
My first time was full of strife and headaches. I've since done it many times and believe me, it's actually not too hard!
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