Artificial rooftop pool for rescue ducks (Calcutta)

I received a duck as a "gift" from one of the local workers known to me.
The next bit will be a bit graphic so be warned.
It was badly tied up. He apparently caught it from his village side and tied it up with rope. And left it in bag for about two days till it reached me. Off course it was a surprise to me and I certainly never asked for it nor did I have a clue this was about to happen in my life. When I saw the duck first I could literally see the misery and instantly opened the ties. He was a male mallard and has beautiful colours (will upload his pics and the enclosure I have made for him on my album). I gave water which he drank like there was no tomorrow. Was shivering, didn't stand up for three hours. I researched the Internet to find what I could do but in my mind i had decided I would certainly help him. He also had a left wing injury which I plastered and it has healed to a great extent now.
I had an old rabbit cage on our roof top. Ordered straw and bought a couple of baby bath tubs.
I don't want to release him in wild again as there is no guarantee he won't face the same fate again there...on the contrary he certainly won't if I keep him.

The idea is to make my rooftop into a best possible setup so I can provide as close to a natural ecosystem to ducks as I can so he can stay happily.

I am looking for a pool which I can put inside the enclosure so he can actually swim in there. I have also planned to get another female (probably a Pekin duck) as ducks are not solitary creatures and live best with a companion.

Feeding him with fish bits, rice, vegetables, left plants and grit inside the enclosure.
The current tubs I have placed are deep enough for him to bathe himself and dip his head completely. But he will be happier if he can swim properly.

Please take a look at the pictures on my profile. I will upload pictures and videos on my Facebook as well.

Any ideas with the whole project would be greatly appreciated. Please also help me with the arrangements I can make for him to swim. I am thinking of large old sinter tanks any one could help me with.. Preferably the white plastic rectangular ones or may be buying the artificial pools from ebay but not sure if they will last 24/7 rough use.

Please advise.

Kind Regards,

Jeet

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