Just google plastic water tanks.  Lots of mfg and suppliers selling any size you can imagine.

Any reseller selling horse trailers or trailer accessories, probably sells High Country Plastics.  And all their models are too wide for your rack.

Look at the dealers/mfg that make agriculture, chemical and industrial tanks. You can find almost any size imaginable. Of course the price is probably a little more.

Here is an example.

http://plastic-mart.com/class.php?item=584

Make sure the water tank you are contemplating is UV safe. Some plastics will be damaged by direct sunlight, and need to be coated for any longevity.

Gard