Update: Since I've moved from SketchUp to Fusion 360 for my modeling, I've been going back through older designs and redoing the good ones in Fusion. The basic design remains the same, but now there's a parametric model available. The original STL from SketchUp remains.

I've got the Park Tool FR-5.2G cassette lockring tool for removing and installing cassette lockrings. The 5mm guide pin is nice to keep it aligned when using it on a quick-release style hub, but what about the newer 12mm thru-axle hubs? You could buy the Park Tool FR 5.2GT which has the 12mm guide pin, but then you've got two separate cassette lockring tools.

Instead, you could print out this handy little converter, and be able to use one FR-5.2G for both types of hub. It's designed to fit nicely inside a 12mm thru-axle and fit over the 5mm guide pin of the lockring tool with a tighter fit at the end of the tool to keep it from accidentally slipping off. It should still fit loose enough that you can easily remove it by hand. Depending on the accuracy of your printer, it may require a little bit of sanding afterwards, but it should be good right off the bat.

It could be longer, but I don't feel that's necessary - the current length is plenty to guide the tool without any slop. If you really want it longer, the 5mm guide pin is 55mm long, and I made the part 40mm long. Scale it in the Z axis by 37.5% in your slicer and you should wind up right around the same length as the guide pin, but I don't think it's really going to buy you anything. Also, since the end of the nose is designed to be a light interference fit to hold it in place, you don't have to push it all the way into the tool and you'll get a little bit more engagement that way.

Works with Shimano or SRAM lockrings, 11 or 12t, as well as SRAM 1x cassettes.