I solved this problem with a part from a 70s era BMW motorcycle.
61 21 1 233 028
Google this number and you can see what it looks like. 6mm threads. I've used this item in several situations over the years. One was the exact situation the OP encountered. Should be under $5.
BMW used 5 of these to fix the battery bracket to the frame.
There a listing on eBay if a BMW motorcycle dealer isn't convenient, though freight is 3.95. I'm not sure if BMW car dealers can get it, though a car dealer in Virginia has it, for $24.