you need a good bit of batting for this...maybe 2 layers of inner-tube (i was lucky to have a piece of 1/8" thick fake leather handy)...cut a piece 7" X 2 1/2 ", lay it down on your top-tube.
re-install the tank, centering the bolts/brackets on the padding. don't be afraid of it, tighten it down pretty good on your padding.
the padding will start to compress, then you'll "feel" when it's had enough & you're stressing the bolts. the metal straps will take shape, stop when they do, try to move the tank, tighten very-slightly more if needed, repeat.