How much space do you need?

If you don't want anything too big, then I can thoroughly recommend a Wolfman Enduro tank bag.

At 6 litres, it ain't the biggest, but it's shape is designed for enduro and dual sport bikes so it fits the sloping tanks of the XTs perfectly. Best thing about it, besides the quality, is that you can still access the filler cap without the merry dance that is faffing around with zips, straps and velcro etc.

http://www.windingroads.co.uk/shop/product.php/7/wolfman-enduro-tank-bag

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