Aside from welding, there are a few other methods that you can use to fill in holes in metal.
Epoxy, a chemical adhesive, is considered an alternative to welding due to its strong adhesive properties. It is most effective when used to repair a small hole. When using epoxy, it is vital to properly prepare the area before applying the epoxy and sealing the finished application.
You can also apply a patch of sheet metal on top of the hole. This is a more straightforward technique than epoxy, and all you need is a sheet of base metal and nails to attach the patch.