I have a Gen 1 KLR and bought a Wolfman Small Expedition Tank Bag
I've only used it a couple of times being new to the sport but I am happy with it so far.
It is big enough without getting it the way of anything.
It's waterproof and has a map pocket for maps, i also stick my phone in there too as it runs mapping software.
It has a small removable zippered pouch under the cover too which is handy for small things.