Oh, here's the part # if anybody's interested:  SUM-290101

That plastic tank is 13" wide x 13" long x 8" tall.  With a 2" sump which may or may not be included in that 8" tall.

13" wide will stick out two inches each side past the fender rails (they are 9" o.d..)  Stock pillion is 11 inches sissybar to driver seat -- you'll have to sit 2 inches further forward, or lose the sissy bar.

If I were to go the auxilary tank route I would still opt for a Tour Tank.  The two gallon is only 8" diameter and 12" wide and would attach to/behind the sissy bar quite nicely.
The 10" x 14" four gallon tank would be, IMO, a bit too large for our little bike.

Two gallons extra would be 4.8 gallons total, 288 miles at 60 mpg -- which will take me into Downtown Las Vegas.

The object is not to not stop, but to not have to stop for fuel.  As in "Let's run to Las Vegas via 29 Palms, Amboy, & Cima.  There is gasoline in 29 Palms, and again at the Stateline, and 140 miles of lonely roads inbetween.