I am about to go on a bit of trip soon, so instead of lugging 20l jerry cans of water around, i decided to put in a 58l dolium water tank. At the same time creating a flat floor the whole length of the tray. Here is how it progressed through the build.
Testing the tank for fitment
Making the frame up. I borrowed this idea from a fellow NTN, who made some draws up out of this "Connect It" tubing, which is really easy to work with. I also added strength to the joins by adding steel brackets in the corners. The frame is very light, but very strong also.