Grant Erwin writes:

I find I have no idea how to shape e.g. a 4" soft aluminum circle so it
will lie close to the OD of a rounded 3D surface like a tank end.

Cast the conjugate shape against the original with Bondo. Or epoxy plus
quartz sand instead of Bondo. Use that as a one-time spinning mold. Or
make a 2nd cast (positive this time) and use the pair in a press to form
the sheet.