I went about it another way. I got a plastic euro crate / parts crate thingy, the tank is a perfect snug fit. Then put a couple of D rings on the inside of the tube and connect the euro crate to the D rings.
The crate isn't held down to the floor, but it doesn't really move around because of the weight. There was room to fit 4 tubes at the handle end of the tank to act as fishing rod holders.