I'd carve a wooden form like the inside shape you want it to be ,
a polished face of a hammer wont make marks every time you hit it..
trouble with aluminum is It's hard to anneal , and that reduces the work hardening that is natural to a reshaping .
but it can be reshaped a small hammer blow at a time , till its shaped like the form you made underneath it.
this is a way to narrow the width to not wear the paint where the edges touch the Paint.