I use plugs that I get from the local sheetmetal shop. Usually I get sizes from maybe 1/4" to 1/2". When the metal shop punches holes for whatever they are doing these just end up on the floor. Whats neat about the plugs is they are exact in size, say 3/8", and they are peaked a little bit in the middle from the die they use. If I have a hole to fill a little smaller than 3/8", I'll use a uni-bit to make the hole 3/8" then use a rat tail file to make the hole just a bit larger so the plug will just snap in. Then because of the small peak in the middle of the plug, I'll use a hammer and dolly and flatten it out and it holds in the hole real tight. Then just weld. Hope this helps..... Bob