The steel drum thickness was specified to be >0.7mm, which is nearly half as thin as the typical metal sheet used in handpan production, which is 1.0-1.2mm.
I started sinking the surface with a rubber hammer, which turned out great as to not possibly overshoot the first few hits, and effective at first—as I managed to sink to about 60mm of depth in the first hour—but after that, the force of the hundreds of hits had already substantially work hardened the surface to levels where the rubber mallet was simply insufficient.