Just a thought...
You could use a piece of scrap 2"x4" a bit shorter than the tank to give the tank somewhere to sit.
Hold it against the side of the top bar so that the top of the 2"x4" is level, and just above the top tube.
Trace the curve of the top tube against the wood.
Cut the wood along your line, and set the cut-off on top of the tube.
Sand and shape the wood to fit the top tube and underside of the tank as best you can, and try to create a concave groove in the underside to sit on your top tube, paint/stain/seal the wood.
Put a bit of rubber between the wood and both the top tube and the bottom of the tank, and fasten the tank down.
Odds are that the wood itself will be completely hidden by the tank.