Neils, the dutch water bottle fits the mug, both the old 850ml one and the newer 1l bottle. Endys sell them, but you should be able to source on at home very easily. I've got 3 of these set ups myself, as well as a crusader and numerous Ospreys, a Tantoka mug and nalgenes.
Thanks for the advice, but I don't really need one anymore now. I really like the crusader setup, but I need to figure out a better carry system. Like maybe a strap over the shoulder or something.