assuming you  dont have the oem "luggage rack" and you do have the backrest..there is a  bolt on each side of (inside)where the backrest meets,mounts,to the side rails..you could fab a piece of  flat stock bent on each end with holes to attach with the backrest bolts kind of a upside down U shape  thats flat across its top, angled back to attach either the tank or a platform (sheet stock)
no welding necessary  parts available at your local hardware store...if you can visualize it you can build it..
may help to make sample from old shoe box b4 sacrificing
metal...

Cool, someone to try my ideas.....

Starting with the stock, mostly, rear end:

lose the stock taillight and go with a 5" wide catseye.  Looks like it might simply replace the licence plate lamp:
 
Then mount the tour tank to the sissybar itself (didn't photochop the straps, but you get the idea.)  By removing the stock taillight you get a lower and cleaner look:

While they suggest mounting on a luggage rack, I see no reason I couldn't simply make a mounting plate that bolts to the sissy bar.

Tank and bracket under 5 pounds.  2.25 gallons of gasoline = 13.5 pounds.  Less than 19 pounds total.

mick wrote on 12/31/69 at 16:00:09:

got mine  from the dealer when i bought the bike(06 check with dealer for your year)  no mods bolts(clamps) to sissy bar, put it on myself biggest pain was adding side rails but if you have sissy  bar you already have the right side rails..dont know about mods for tour tank should be minimal if any..