Since no one else has ventured a thought on this.  I'll try.

Those triangle water tanks hold about 30 gallons of water. Water is a little over 7lbs per gallon if I remember right. So it will weigh over 210lbs. That's heavy enough that you should weld a sold support bracket under the tank to hold it. I wouldn't try to support that with straps.

I'd problably have a weld shop weld a tray across the front of the trailer bulkhead. Slip the tank into the tray and then have some kind of retaining tray that you bolted in place across the top edge. Just find a shop that is good at welding aluminum.

Personally I think this will be more trouble and cost than buying a new plastic tank to fit a roof rack or truck bed.

ISHOM, 

You've asked a good question and PaintedHorse gave a good answer from a weight and modifications point of view.  The costs of welding up special brackets and then modifying your existing tank could run in to lots of bucks and make it not worth while, even though you already have the tank.

An alternative might be http://www.doubleduty.com/accessories.htm This was discussed earlier last month, and provided some additional points to consider.  For more info on that thread look up #38253, posted on 2006-03-04 for some other considerations. http://www.horsetrailerworld.com/forum/thread-view.asp?threadid=4574

For a cost comparison with other types of tanks, check out http://www.horsetraileraccessorystore.com/watertanks_accessories.htm

Hope this helps, Steve