I have a Gen 1 KLR tail bag that I have used on two KLR's and two Super Teneres and it is still holding up well. Not waterprof, but I store the contents in plastic bags when on a trip and have had no problems. I don't run a back seat and have the rear rack lowered, so this fits perfectly. Has side pockets for my tool kit and a few odds and ends. These are still available on ebay. The Gen 2 KLR bags are available also, but don't fit the provided space as well as the original size.
Edit, this is expandable to about twice the height.