That pic is from a friend's bike, before I added my aux tank. I put my bulkhead fitting in a similar location, but lower and more centered (down between the two seat rubber bumpers).
I pulled the gas tank and looked at the metal plate, but it's pretty crowded down there, with no flat area that I could easily drill into. So I went into the tank itself at the back. As long as you make sure there are no interference problems with the fitting or hose and also use the correct plastic sealing washers, it'll be fine. Oh, and do drill the gas tank hole with a proper step bit. That's the easiest way to ensure a clean hole that will seal well. To drill the hole, mark it carefully
then drill a pilot hole from the outside before switching to the step bit. Obviously the tank should be empty, and you can remove the metal plate to get your hand up in there with a cup behind the hole. That will catch most of the plastic shavings before they get sent all over the inside of your fuel tank. Then you can get a socket in there to tighten the fitting up properly.
Other than that, you have to decide what type/size of fuel call and where to mount it. I used a Jaz 5-gallon fuel cell
from Summit Racing. That was the best combination of size, shape, and price. I mounted it in place of the passenger seat using some custom brackets that kept it low enough that my 49L case would still open. I made the straps to go around the fuel cell as well, although I'd recommend just buying the pre-made ones that Summit offers.
Do get the foam inserts which minimize fuel sloshing while leaned over in a corner. Also, you can use the same quick disconnects that BMW uses for the main fuel line, but get them in chrome-plated brass instead of plastic. And while you're at it, replace the crappy plastic connectors that BMW puts on the fuel tank with the metal ones as well.
I used an inline ball valve to isolate the fuel cell from the main tank. Ride until the light comes on, reach behind and flip the lever, wait about 20 minutes, then you have a full tank again. It does take a long time to drain the entire 5 gallons into the main tank, but at that point you're just riding so it's no real bother. And it's pretty cool seeing the fuel level rise as you're riding along.
I also made up a simple back rest out of some dense foam and vinyl covering. And I added two 3-liter water bottles as well. Add the small bag attached to my handlebars that contains granola bars and other munchies, and I can easily ride out the entire 11.3 gallons.
With my ~40 mpg average, I can get about 450 miles between fill ups. You could easily get over 500 miles if you ride a bit more leisurely, but where's the fun in that?
'13 Dark Graphite Metallic K16GTLD, 24K miles and counting...
'09 Magnesium Beige Metallic K13GT, 60K miles miles and counting...
'02 Mauve Metallic K12LTC, 106K miles and sold
BMWLT#145, IBA# 366, MOA# 111996, SCMA# 24032
All lower 48 states plus Alaska on the K13GT in two weeks . . .
Some people see the gas tank as half empty. Some see it as half full. All I care is that I know where the next tankful is coming from...