Some folks use a locking 'trunk' available from most motorcycle shops... just mount it to your rack... they can be heavy though.
like this: http://www.motorcycletrunk.co.cc/re..._TRUNK_BLACK_LUGGAGE_TOP_CASE-B001PMI3JC.html
like this: http://www.motorcycletrunk.co.cc/re..._TRUNK_BLACK_LUGGAGE_TOP_CASE-B001PMI3JC.html