I also recommend the Ortlieb. On the days when you don't happen to need the big camera, you can use it for all your other stuff.

The Ortlieb mount is rock solid. It'll be far, far better than rigging up your own bag.

On a side note, unless you're routinely making 11" x 17" prints, a DSLR is overkill these days. Mirrorless and even high-end compacts are more than capable of handling anything you'd want to photograph while on a bike or on tour.