My personal favorite:
fits between the filler caps very well, does not extend too far back so when standing it does not interfere as much as the KTM bag. rain cover is extra, however the cordura is slightly resistant to water, its the seems that leak.
the black expedition large is another option that fits well and is mostly water proof.
next option, is this product:
these are some top shelf quality built products. I believe more intended for the hyperlite crowd, but very nice "in between" size. slightly larger than the Wolfman expedition "small" and the explorer light