have you considered using one of those collapsing plastic water bags / jugs as a tank? That corner storage area is a real odd-shaped space with a lot of volume, especially after the jack is out.
I think it would hold at least 6-7gal in that back corner. Could even rig a tap out the stern end of that body panel, by the rear hatch of the vehicle. I'd be a bit concerned about pesticides and plastic and my potable water. You can get a similar polyvinyl sink drain downtube for just a couple bucks at any hardware store. Or new PVC for a similar low price.
volume of a tube goes up pretty sharply as the dia increases, pi x r squared x length of the tube, divided by 231 cubic inches per gallon. They're a nice way to store.
You get about 2/3gal per foot of 4" pipe and about 1.5gal per foot of 6"
Saw a real nice roof rack shower supply done with 4" x 8' ABS, the guy even pressurized it with an air fitting. And the black tube gets hot in the sun just sitting out.
www.roadshower.com
You made your storage box a foot deep? (roughly the height of bottom of that factory compartment) I'm curious what all you've got in there. I'm planning a two-drawer and a multi-flavor power module under a deck that's no more than 8" tall (need headroom for big dogs), had thought that was a lot of volume for regular gear storage. In fact figured I'd have one drawer mostly empty to use for range trips. Not meaning to be nosey or contradictory, just wondering what all you put in such a volume, that you ran out of room? Does it include all the camping / expeditionary gear as well as general gear and tools?