Originally Posted by
ze_zaskar
... ... ... When travelling I only intend to strap a drybag on top of the bag. ... ...
I assume you meant strap it on top of the rack, not bag. If you have no panniers on the rack, you need a fairly wide platform on the rack to support a drybag that you strap on top. But, if you use the panniers on the rack, that makes rack platform width less important as the panniers will help support the drybag.