I'm trying to fill up a bunch of the unused holes in my inner fenders/ engine bay to clean up the engine bay since the harness is going to be tucked.
Most of the holes are too big for just MIG welding shut. I was thinking of trying to find a sheet metal punch tool that would make intact tiny slugs to weld in.

Does anyone have tips on how to fill the holes easily? Or what type of punch might give me useful slugs?

What about using a copper welding backer ?? Just hold the copper behind the hole and weld it up...PS..Practice makes perfect there's no sheetmetal screw/bolt hole that's to big or small to be migged...If the holes are relatively small a tiny slug might not give the weld penetration you'd want with such a small area you're better off just doing it...

Second tip...Use a 110 volt welder with something close to a .023 gauge wire to control the heat...

How big are the holes? I can easily fill 1/4" holes with the mig. If the metal is thin or the hole is larger than 1/8" or so I use copper backing.
I recently upgraded my mig to a Millermatic 211. Awesome machine to say the least.

Copper backing is the way to go. I use small length of 1/2" copper water line I had laying around after plumbing work. Flatten it out and hold with small visegrip.

I've used the sheet metal punch tool method you describe with great success. I shaved about 15 holes in a firewall a few years ago and used the method for probably 5-6 holes that were small. I simply prepped everything and put a piece of tape on the back of the hole, dropped the punched out piece of metal onto the tape and centered it leaving equal space around the edges. I then tacked the piece to the firewall and removed any tape that didn't burn away before I finished welding everything up. I dressed the weld on both the front and back side of the firewall. I've had the thing in primer for awhile and started back working on the cab of the truck this past Winter; I can't even remember where all the holes are at as they are completely undetectable.

I used an air punch/flare tool from Harbor Freight to make the slugs; the tool is $3X and some change.

After sanding the back of the cab yesterday; my hands feel like I used the tool on my fingers. I've got 1-2 more rounds of block sanding the rear of the cab and it's off to paint. I'm so ready to be done with this thing.

Copper backing it is!
Most of the holes are 1/4" but some bigger. I welded up 2 with no backing I just had to do way more grinding to get it flat than I would like. I'm using a gas 220v with .023. The sheet metal is whatever thickness most of the engine bay is.

I have the same tool parks5 mentioned and it's great, I just found it has too small of slugs to be useful in my case.

Thanks folks.

Weld in the head of a plain steel nail (Think the kind you pound into wood to hold stuff together). You can get nails in several sizes, get a nail with the head diameter as close to the same size as the hole is. Insert the nail from the bottom side, hold the shank with pliers to keep the nail head centered in the hole, weld the nail head to the sheet metal surface, cut the shank off and sand/grind it smooth (carefully). Gene

I use one of these - great for holes where you can't reach the back side to hold the copper backer

Durston Manufacturing Co MPWT Magnetic Plug Weld Tool

Cool tool!

R.