That tank is spot on, ive ordered a throatless shears as i thought it was more important to get the panels cut all flat rather than struggling with a pair of snips. Im going to try making a steel tank first,until i can get to grips with thin alloy. So the edges were formed just from hitting with a hammer ? did you use a bag at all?
What I do is to cut out the shapes first and fit them together before I roll the edges.
As you roll the edges it makes the flat patterns distort, but if they're pulled in the direction you want them to go first, the whole thing pulls into shape as you tack it up.