I don't think it's necessary on the bike. The Arkel bag already has four hard-plastic sides and a plastified top (mapcase). I assume you'd put some padding at the bottom so it takes forever for water to get in. I use the same handlebar bag with the rain cover and my camera never got wet... until I took it out for a shot. Remember that whenever you take the camera out and put it back in a waterproof bag, you'll put it back with all the ambiant moisture and it'll bath in that air.