I have the short 2 1/2 gallon tank. Original rubber band was a short band that looped over the bracket that robs ss shows in his diagram, then went under the cross tube and up over the tank hook.
After the band deteriorated, I used a thick section O ring that worked similarly. I found a part number somewhere for the Atlas tank rubber band, bought one from AN, but found it was much too long ..... perhaps it was for the larger tank, or perhaps it is meant to secure the tank in the manner Bodger suggests.
I now have a rubber band, obtained from ???, that hooks up in the way the OP cites .... that is over the tank hook, then under the cross bar, crossing from front to rear, then back up to the tank hook. Works fine but is not concour's original.
Slick