Back in 2011 I built a tank for a bike doing what you have in mind. I covered a steel tank with foam then sanded the foam into the shape I wanted then fiber glassed it. It's a time consuming way to do it but ay least you get the shape and size you want.
The tank build starts on page 11 > http://motorbicycling.com/showthread...=35675&page=11
Maybe that will give you some ideas. Good luck.