I used an angle grinder with a cutoff wheel. A cut off wheel in a circular saw works as well and the larger disk makes it easier to cut straight.
After I cut off the top I ground down the edge with an angle ginder and then sanded with a belt sander. A little hand sanding and it is golden.
On a side note, if the Weber lid fits over the fixed drum lid I would cut the lid inside the bead from the top.