Also, using bungees or straps to hold on the drybag, be extra careful of them being very secure. A loose bungee or strap in a rear wheel could really mess up your day. I mention this because when we have panniers on either side of a rack, and stuff bungeed to the top of the rack, if any of the securing stuff comes loose, the panniers are there to help stop it going down towards the wheels from the sides. Just keep it in mind and use common sense.
Yeah, after reading some stories about accidents related to bungees, straps seem to be the way to go. Thanks for the piece of advice!
However, I found some bungees that come with these hooks:
Robot Check
Once attached, they should stay in place.
I will upload some pics when setting off for a mini-tour (if I find some time..)