It sounds like you want something to keep inside you vehicle, but if you are not. PVC pipe can make tanks of different sizes, and RVer's have been doing this for a very long time. I'm working on one to mount on my roof rack for solar hot water (painted black) using a 6" pipe at 6' long, and will hold about 9 gallons. If I go to 8" I will have 15 gallons.
This isn't a new concept so there are tons of DIYer's out there who have don it, just do a search.
Had to chuckle about post #6, handling the flexible bags of water... is entertaining”, somewhat like fishing andholding an eel or something... while they can certainly form-fit your travel space a lot better than a rigid container, they are a PITA to work with. Post #17 is the one I use, its a compromise between the size and rigidity of the WM 5-7 gallon “cubes” and the bags.I just look for the BPA free containers.
As an alternative tI did see a “Lifeunion” collapsible 10liter water container on Amazon for $11 that was well rated, 4/5 with 70 plus reviews.
am I the only one who went "what the helz is that? Like did Cornelius from Planet of the Apes invent some sort of water container? anyway........ for the others who went "whaaaaaaaaaaaaa?? " https://en.wikipedia.org/wiki/Cornelius_keg
I'll just update my suggestion by saying I've never had an issue with the disposable 2.5g I posted earlier until today I'm sitting in camp on day 2 and find all 3 I'm carrying busted with 7.5 gallon of water in my back floorboard and only 1 gallon jug left
Hard to beat the $15 Reliance container for the money. Holds 7 gallons, comes with it's own spigot, is reasonably durable, and it fits inside a milk crate that can then double as an elevated base for the water jug.
I love the portability of the MSR bags. The biggest issues Ive had are the Water is never cold!!!!! The black bag keeps it warm/hot and in the summer is not fun to drink! Ive used plastic jerry can type water jugs and almost all of them have worn holes in them from rubbing and miles of dirt roads. I have been using 5gal stainless steel Cornelius soda kegs. They are super durable and i have made fittings to work with my camp sink. I use a bike pump to pressurize the kegs and push the water out. I have even made a heat exchanger that i dunk in my boil pot that will provide hot fresh water as well. It works really good and is super durable. And yes a bit heavier then plastic.
I dont have a write up on it. Its super simple though. I go out with just a boil pot to heat my meals that are in sealed boil bags, that I prepare before I go. It allows me to reuse my boil water the whole trip, conserving my fresh water supply. My clean drinking water is in my 5gal Cornelius kegs. when I want hot water to wash up or for beverages I have a quick connect to my copper coil/heat exchanger that I submerge in my boil pot after or during meal prep. I turn on the water tap and the cold water goes through my heat exchanger, heats up and out my tap. I have put it on a regular faucet that adds cold water so you can have just the temp you want, buts its a bit more gear i need to bring so most of the time i leave that part at home.
I use 3 litre juice bottles; they have a handle so it's really easy for the kids to carry, refill and use; this means no (or at least much less) spills when using. They also pack perfectly in my Dometic fridge and don't have a plastic after-taste. I prepare them in advance and transfer them from the house fridge to the truck cooler (or new fridge!). I use the same as my emergency water supply at home; there are always three in the fridge and three in a cupboard; the water is used and refilled to keep it relatively fresh.