Long range tanks
As Scott pointed out, I did something like this on my RV-9A. I wanted more fuel capacity but not twice as much. I looked at what others had done and decided to do it a little different. I created a single piece leading edge skin and expanded the tank out 3 additional bays on each side, adding 18 more gallons total.
The process to do this is simple, HOWEVER, bending the aluminum leading edge skin is nearly impossible without the right equipment. I made a homemade contraption that I finally got to work (scrapped one skin trying) but I really can't recommend it. If you can find someone with equipment to bend the leading edge skin, the rest is easy. You'll also need someone to bend new back baffles.
If you want details, email me or look at my website, I have a blog dedicated to the fuel tank construction.