I have a fuel cell on my pillon, but it's a little bigger than what you describe - just over 4 gallons. It was actually made for a K1200LT to bring it exactly to the IBA limit of 11.5 gallons, but I was able to buy it from someone (with a K1200LT!) who decided to go a different route.
It's mounted on a Jesse plate and is quite secure.
http://www.sampson-sporttouring.com/fuel-cells.html is the manufacturer of my cell. There are others.
Make sure you ground and vent your cell. Drilling into the flange on the side of the fuel cell is a little scary, but take your time and it'll be okay!
I have a quick disconnect on the line from the cell to the flange so I can just disconnect the QD and take off the Jesse plate and it's off.
Also, you might be interested in a custom fuel cell cover by Linda T. I have one on my fuel cell with a couple pouches on it and I LOVE it.
http://www.customtankbags.com/
http://www.tourtank.com/ might be the company someone else mentioned. I love this one because it looks like a little keg!
Good luck!