I agree about being careful how you mount the tank, learning from experience designing and building drill rigs for the mining industry. The key is to provide some support that overlaps perpendicular edges..... in otherwords, block at the corners or edges, and not in the middle of any wall of the tank.
Also, be careful about hard mounting with any kind of tab that is welded or fastened to the tank. Mounting tabs on a fuel tank are in my opinion, a bad idea as these are areas prone to cracking and failure. I have seen instances of tabs ripping the side of a fuel tank wide open. The key is to grab or capture the tank, not fasten it down.
Oh, and don't forget a layer of abrasion resistance (such as rubber or felt) to keep from wearing a hole through the tank while you are out there pounding on the waves!
