trawl Ebay for Top Boxes you might find one on their, try to get one with its mounting plate as that will offer some hope of being modded to fit the rear of your bike (no idea what an XTR has on the rear for rack mounting). I modded the HB box and rack mount from my old XTZ 660 to fit my XTZ1200 with a drill and some bolts /spacers..
PS you'll probably find that the max rear load rating is 10Kg or less so bigger the box isnt necessarily better and overloading could lead to lots of other grief the best of which might be a high(ish) speed weave. Alternatively Ortlieb waterproof bags (or similar) work a treat and are easily strapped on/off and carried (just dont expect your dirty keks to come out smelling of roses in warmer/sunnier climes).

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ex XT660X Tenere, now XT1200Z Super Tenere