I bought a Rotring 600 and loved the entire experience, except for the looseness of some refills in the tip. I created a simple 3D-printed solution that requires a little bit of post processing to snuggly fit your favorite refill. The small hole is modeled to the exact diameter of the smallest refill I have, which happens to be a Monteverde gel needlepoint, but it will likely need to be delicately widened using a file, etc., to get an exact fit unless you modify the CAD file to match the accuracy of your printer and the size of your refill. I'm fairly confident that you won't need to modify any other geometry, unless your printer is poorly calibrated. I'll update it with any requests or feedback.