Take the gooseneck off of a bike and go to your local high school skill center. Some kid needs to learn his craft.
It's a round wedge on a shaft, extending through, and bucking up against, an angled tube 60% as long as the barrels you wish to raise dents from. It's not a complex tool to make.
The plannishing of the barrel is where the skill comes in. Get a plastic hammer at Harbor Freight.

Out there doing it best I can.