Bike mounted camera bag
Ortlieb used to make two camera-specific bags that you could probably keep inside their handlebar bag, the Aqua Zoom and the Aqua Zoom Plus. These are now discontinued but you can still find them for sale.

However, the Ortlieb Ultimate is a great handlebar bag that snaps on and off the handlebars, and also has a camera insert. I highly recommend the Ultimate, although the mounting is a little tricky to install.