so far i have seen a few good posts some truly retarded ones and a couple that go so far off topic i would have to think that the person typing them is either severly handicapped or suffered some nasty head trauma
someone mentioned kiddie pools or stock tanks.... perfect for rays, also you could do something close to the mandalay bay's set up with some decrotive blockwork and a pond liner, the filtration on the other hand is going to be expensive to keep something like that, also you have to deal with 10x the evaporation as with keeping a tank so an automatic top off set up would be ideal for keeping salinity levels in check. i would recommend setting up a reef tank and running it for a couple years successfully before you try something of that magnitude, it will also give you time to save up the money you will need for the filtration and protein skimmer.
side note: if i remember correctly the mandalay has signs posted about not sticking your hands into the ray pond with all the bacteria and oils on the human hand and with the amount of people that go through that room every day if they all put there hands in it would be detrimental to the fish.
