Late last year after I smuggled some Nido back, I did a trip with muesli and Nido - I portioned my muesli into a regular small size ziploc, with the Nido inside in a ziploc "snack size" - the ones about half the size of a sandwitch bag - they aren't particularly tough, but don't need to be for this. I would then just add the water into the snack baggie, zip it up,
and (carefully) shake/knead a bit in my hands to break up the lumps and mix well, then crack the snack baggie and dump it into the main ziploc with the cereal. Bag is really
small and light, and means I don't need a seperate container to mix the Nido in.