I found just the right one - at the local motorcycle shop.
I stopped in at Folsom Cycle (my favorite local shop) to see if they had any other options. First they showed me one of those Moose Racing ones as has already been mentioned here, that is slightly larger than the one I already have on the back of the bike - too small. Then they showed me a huge yellow pack that might work on the KTM for a week-long trip - way too big for the XR.
Finally, they showed me one of these - perfect.
And because they had lost the straps that allow you to turn it into a backpack, they knocked twenty bucks off of the price. I'm happy now.
I just need to load all of the right stuff into it and I'll be set to ride out in the middle of nowhere, far from civilization. And it will be easier to make sure that I bring the right stuff every time I'll need it. Thanks to all for the suggestions.
Here's the full description: