Here is what I would do...but keep in mind, I go a little overboard.
Either use the strapping above or (and this would ne my preference) use thin strips of steel (2) that will reach around the tank. I would go with abou 3/4" to 1" wide at 22ga of maybe even 26 ga.
Get a "U" bolt at the hardware store- not a muffler clamp, too big. Get a "U" bolt that is just big enough to go around the seat post. Face the "U" bolt up, that is, with the close end down and the open end up. Below is a rough drawing of what I mean. Put the plate across the "U" bolt and tighten it down.
Get the 2 metal strips and bend them around the tank so that one lines up with one side of the bolt, and the other the other side. Then overlap each strip so that the ends cross over the bolt. Drill a hole through both ends, get a washer and nut, and bolt it down on each side.
Again, refer to the rough (very rough) drawing.