Big white plastic jerry can (25 litres?) in the boot of my LWB Shogun. I've got a sturdy boot liner with a decent lip to contain any spills, but fingers crossed I haven't had any.
I know someone who made a bracket for a jerry can and fixed it to that shelf bit in front of the rear ramp. It seemed to work well.
I just use a 25 litre plastic water carrier, and either use a spare leg strap to attach in the front of the trailer, or yuk it up in to the bed of the truck I use to tow with. ETA I used cable ties to make 2 loops through the metal bar in the front of the trailer, and attach the leg strap to those.
Like everyone else, just put a water container in the car (use the warhorse one linked above) or use bungee straps to attach to shelf in front of trailer
Large squarish water container in the nose of the trailer either bungeed in place or secured with ratchet strap to the frame of the trailer to stop it shifting