After a 3 days Haik with the scouts (you can find the trip on www.outdooractive.com) we finally had our last camp at the big woodbridge in Kössen. There we were picked up by a rafting team, which brought us - 30 scouts, who all had arrived from different haiks at this meeting-point. Because of the long dry periode of the weather the water was really low. I think with a good technike you can manage the river in an onak, but on one or two sites a sprayhood would be fine.
So I am still thinking about building one.