just remember to keep it in yer outside locker or passenger seat, dont strap it up outside, thats how they go all green and dirty, use denture cleaner or similar to keep it clean
Mike-C:
I always buy water in them 5 litre containers from Tescos or where ever, but mostly re used them 2 pint plastic milk cartons, found them ideal to shove in the door pockets and easy to fill at anywhere with a tap.
me too 
5litre’s of water for 98p 
just re-fill it when needed