The Nashbar bag that goes with that rack has little, side pockets that hang down on either side of the rack like mini-panniers, so it probably wouldn't meet the OP's criteria. It also was not waterproof. Perfect fit + cheap + waterproof seems unlikely. I would find a bag holds your cameras and is waterproof, and then find a good way to secure it to the rack. Limiting yourself to something with the exact footprint of the rack seems like it's going to eliminate too many otherwise good options. I believe I have used mine with a soft-sided cooler, though. Not waterproof, but about the right size. Of course you'll have to DIY a mounting system. I just bungeed it on to my rack.

In the past I've considered getting an appropriately-sized Pelican case and drilling holes in the bottom (underneath the liner) to attach it with clamps to my rack. A soft sided case with your camera's could fit inside of that and the camera bag would be easily removable and would not have to have any bike rack mounting attachments. The Pelican case, of course, would be pretty much permanent, or at least not something you'd want to regularly mess with.