How far do you want to go?

[flecc]

Those batteries prevent you attaching panniers to the pannier rack. - - - - - - - - - - Keeping the batteries in the panniers keeps them safe from bumps and jolts plus leaves you free to use the rest of the storage space in the panniers.

That's what I prefer as I posted earlier. I suppose one possible advantage of the custom rack mounts is that one could run all the batteries in parallel for even more range/performance advantage, something not really practical if they were carried in panniers.