Many people rely on VW's expensive 1-liter bottles because, for example, the filler neck is often located in a very inaccessible position, and you need the bottle nozzles (e.g., in the Sharan).
This evening, I noticed that an Aral gas station had apparently "displayed" empty 10-20 liter canisters in a wire box, presumably for nighttime collection.
All the canisters still had their long spouts clipped into the designated slot.
Possibly, you could take one of those to build funnels.
But make sure to clean the nozzles thoroughly. This stuff crystallizes very quickly...
"What's the problem with filling the Sharan with water using a tap? If you're not particularly uncoordinated or lacking in mechanical aptitude, it's quite doable... However, if the trunk is full and you need to top up the water in an emergency, then none of that helps."