[Synchro909]

I once repaired a pair of Chrysler headlight shells. They turned out very well. I filled a box with brick mortar and scooped out a hollow large enough to accommodate about half of a shell. Once that had set, I poured Plaster of Paris into it and after spraying the UNDAMAGED part of a shell with WD40, I pressed it into the plaster. Once this set, I had a perfect shape to aim at. By GENTLE and judicious use of pieces of wood etc (like those mentioned about by others) and a small hammer, I gradually closed in on a good shape. For those very slight imperfections, I used a piece of broom handle with oil on the end and rubbed the interior of the shell till it conformed with the shape of the plaster mould. Very time consuming and I don't think I'd bother for a pair of shells I could by new for a reasonable price.
If, as you say, these are to be painted, a little filler applied to a roughed up surface (for grip) would save many hours of work for no real gain. Remember, this is all time you'll never get back!