Front-wearing camera bag
I have used a Lowepro top loader pack as a chest pack and attached it to the load lifter straps on the backpack, or d-rings provided on the shoulder straps. This will hold the camera and 1 lens with accessories in the front pocket. I've used a lens case on the waist belt for a second lens. You may also want to look at the Clik Elite packs. They have a chest pack designed to work with their backpacks.