I would use the bottom of your drum to form it. It should have a flatter more easier to work/clamp with raised flat flange than the rounded top part.
Leave the piece full length (it will be easier to bend than a short piece) and your guaranteed to not wind up too short.
1/8 inch should not put up too much a fight.
Lay the drum on the ground with the piece clamped and roll the drum to help you bend the metal. Your weight on the drum while rolling it will be easier than trying to pull it around by hand alone.
This is on the outside of the drum, once its rolled you can then work it smaller to fit into the top of your drum.
UDS, XL BGE, and a pane burner.