I have a 1.5 gallon gas or a 2 gallon water I mount on my 990 ADV. I use the T mount set up to a custom bracket top mounted on my Touratech pannier mount. In certain ways the 2 gallon is better than the 1.5 as it is flatter and makes for a perfect platform for duffel bags and such. Of course the 990 is a bigger bike than the DR 650. Also, side note, the water specified container is cheaper than the gas, but they are exactly the same plastic. If you don't mind it getting discolored, I believe the white one is half the cost of the red one.
'06 Beta RR 525 "Red KTM"
'02 GasGas EC 300..."The Smoke"
'10 KTM 990 ADV R "Brutus Beefcakes"