An option for carrying the 1.5-litre bottles is something similar to Topeaks Modula Cage XL. I have something similar, which, IIRC, came from BBB.

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[utag=413859]base2 my solution for carrying extra water in the outback is Sea to Summit water bladders. It looks like they do not sell the ones I have but other brands do them as well. I find the ones with tie-down loops are the best. I have carried 35 litres at one time using the Ortlieb ones as shown below. I have now swapped them out for the Sea to Summit type of bladder

You can see the Ortlieb bladder (blue bag) on the back of my Salsa Mukluk, and the reason why I have gone over to bladders with tie-down options. Getting it to sit on the rack and stay there was a pain. My XL cage is visible on the down-tube.