I could easilly see a RTC living it's whole life in a 250 gallon pallette tank....
4' x 4' x 2'...nylon (available on Ebay for just under $200.00) In fact this would only enhance the interaction IMO. The problem with keeping big cats in ponds...(besides the question of heating them during the winter)... is that you lose a LOT of that close interaction when they have other fish to entertain them and a ton of room to move around. I'm NOT recomending the pallette tank as the ideal situation but it's certianly possible and far more affordable, not to mention, DURABLE than a lot of the other options.
I have also reciently concidered the, very real, possibillity of combining two of these units into a single 8' x 4' x 2' tank. It's not as complicated or difficult as you might think.
Think about it... a 500 gallon tank for under 400.00 plus filtration, inexpencive lighting and a weekends worth of work.....
Some of you are talking about housing a cat that can live for 20 years or more in an inflatable pool that has, at BEST, a life expectancy of a year and a half?