When I toured on my Nomad, I used to have a T-bag can't remember which one - it had a rain cover and worked quite well for me. I reversed it so it was sitting on the rear seat, attached to the sissy bar. It moves the weight closer to the centre of the bike, and especially if it's a larger bag and you overpack, most luggage racks aren't rated for that much weight.
The T-Bags are spendy, but if you do a lot of touring you learn pretty quickly not to cheap out on your gear.
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