You can buy the weld-on bungs for 3 or 4 dollars each on E-Bay and other places, I’m sure. (That’s what I did).
But, you can also simply buy an 18mm X 1.5 steel jam nut and use that. Right now, for instance, you can get a pack of 3 for $1.00 plus $2.80 shipping on Ebay. Or, maybe your friendly fastener supplier has them in stock. My local auto parts place had them. I paid about $3.00 for one, but no shipping.