Fuel Cell

You could probably just get a new tank for your car, and weld whatever bungs you might need before you put gas in it.
But for a cell, I built a welded together shelf for the cell to sit on, and be attached to, so that I didn't have to cut a hole in the trunk floor, and then build a skid plate for it.
Done that way for the cell in three different cars. (without spare tire wells)
Edit. I like the way MMissle did it too!