Weld Bungs are welding plugs that you can use to, for example, mount the custom oil tank to your frame. There are different types: Coped (with a radius), Counterbored (with a pot hole), Threaded (with screw thread) and O2 Sensor (especially for the O2 sensor). And for that money you are not going to make them yourself ............
These products from Kollies Parts are of a good quality and very affordable.
Do not forget to look at the Tabs ......... these are also very handy chopper parts / bobber parts, just like Weld Bungs.