I've been interested for a while in these flexible water tanks:
http://www.aquaflex.net ...It might even be a good candidate for a roof rack
Water tanks come in many shapes and sizes. Collapsible models offer great flexibility and can be stored in many nooks & crannies where space would otherwise be wasted. But they are very difficult to move/carry when full or partially filled. I’ve been looking for tanks I could fit inside a vehicles wheel wells, talk about wasted storage space. I did find these that offer an interesting option for many vehicle types:
CleanCraft