cool tool, I shouldda had one when it was firewall time, lol. I have several flattened copper pipe pieces of different shapes and contours and a thick piece of brass that takes longer to heat up
it should come with a step drill so the old hole can be cleaned up to bare metal and sized for the round slug to fit well. that way you don't have to custom fit every filler slug, just drill the hole to fit the slug. welders series sells "unholes" for cheap, prolly others do too. I sometimes use a magnet to hold the slug due to no helpers. it only works for the tack because the magnetic field distorts the weld and then there is porosity to grind out and re weld. tack, remove magnet, hammer/dolly to get flat, tack again, hammer/dolly......you know.
I have used a regular uncoated nail sometimes. it has a nice shank to hold while you weld, then cut that off when done. the shank helps absorb some heat too
I have a C clamp that I cut off so it is only 1/2 a C. it has a magnet attached to the cut off part so it can stick to the surrounding metal and not distort the weld so much, then the threaded part can be screwed down to fit whatever height you need. it has uses as well.
https://welderseries.ecwid.com/Unholes-p50202282