without cutting into the backplate you'll be creating it is impossible to shape it in the desired backplate form.
you can't wrap a sheet of paper around a football without fold lines.
you are going to have to cut into the plate and glue it back together, bondo it and smooth sand it.
I did the same with a trash can and it worked ok.
had to cut most of the back out though in order to relieve stress on the other curves in the backplate.
but ah well, it was covered by the jet pack for 80% anyways, so who cares? 
good luck!! and keep us posted, any and all methods on creating backplates for us folks without direct acces to fiberglassing etc are more then just a little welcome 