I carry my DSLR and second lens in an Ortlieb handlebar bag which is the perfect size I find, and very secure. I wrote a review a while back (after more than 10,000miles):

'I have the Ortlieb 5L one. This is great. One piece of kit I’m really happy about.

It’s waterproof and holds a phenomenal amount securely – I use it for my Canon DSLR with 18-200mm lens attached and 10-22mm lens in a padded liner (taken from another camera bag I happened to have). Then I fit in my wallet, money belt with passport, documents etc, GPS, compact camera, multitool, leatherman, maps, journal etc. It’s fairly well packed in and some might say overloaded, but I use it as a camera bag when not cycling as it means I can keep my valuables with me at all times. Even with the rough African dirt tracks my camera is still is great nick.
I find the lid sometimes a little tricky to close and some people may prefer a bag with more compartments / pockets. From using it regularly as a shoulder bag, the top edges are very worn now, so I make sure the camera is in a plastic bag for extra waterproofness.
The way it attaches to the handlebars means that it swings up and down when going over very bumpy terrain. However this is an advantage as it acts to dampen the vibrations of the road/knocks and this is good for the camera.'

Edit: The bag and attachment was still in use at the 25,000km mark. I've since bought another one and that is fine after 12,000km. All with my camera gear and extras in.