I had some crazy ideas about this, what about having a big sheet 2mx2m with the sides raised to stop losing water 2 foot orso off the floor, thats set up in a way the water will only run into a drainage pipe, leading into a tank with some tights/sponge etc over the end just before it runs into the tank.
You could quite easily do it on a shed roof, save yourself a shed load of money on all di/ro resin and all that pooptee.
Failing that go to your nearest reservoir and fill up!