can anyone help?

[thedoctor]

I was driving in london the other morning on the way to work (and yes i do use the gsa for work sometimes) and i guy on a gsa rode pass me and on the underneith of his bmw side pannier he had additional aluminium box that seemed to be bolted on the bottom which seem to add to his storage space, or hold a large water- fuel tank?
Does anyone know or have seen anything like this on the market and where i could get to have a look at one!
I would have asked the rider but he was riding in the oppersite direction
CAN ANYONE HELP?
CHEERS

 

[Gruffalo]

in know that touratech do under pannier wayer carriers but come at a price! but they only fit on the bottom of their own zega panniers,