I measured the bottom headtube and ordered a stainless steel ring to go around it. Apparently, pcbway.com is also able to CNC machine parts for you, I always thought they only did printed circuit boards (PCB). I chose the R430 stainless type as it appeared to have the highest tensile strength. It should arrive within the next 2 weeks. Including shipping it was 30 euro, which is kinda steep for a simple ring, but this way I don't need to find time to go to a local machine shop.