To fill those voids you want copper wool, or a copper scrubber material. Stuff those holes with the material. Then get some annealed iron wire, and wrap a few times tightly around the barrels. Then take large head nails, or aluminum wedges and shove them between the wire and the bottom rib. This makes everything stay in place when you go to the next step.
Flux and solder the ends. The solder will flow into those voids and if you backed the copper in tight enough, it will eventually fill up. Leave a little solder bump where the voids were. When cool, file down the bumps, cut off the wires, polish and blue the barrels and you’re good to go.
PM me if you have any questions. I’ve done that repair several times.