aren't the Ortlieb ones claimed to be dry-bag waterproof?

I don't usually ride with it because it's oversize for everyday needs, but i have a deuter futura 42AC hiking/trekking pack and it works really well. I have to adjust the straps to fit when i'm riding (versus upright). Mine has aircomfort, which is basically a suspended mesh panel that keeps the solid part off your back, thereby letting enough air circulate in there. Pretty standard for newish hiking packs. Maybe one of the smaller Futura Zeros, or the alpine ascent Black Diamond ones would work nicely.