Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:4939954.

Bought our kid a bike for her birthday. I noticed the head tube was missing the rubber cap that closes off the tube. This is potentially a safety hazard so I whipped up this little ditty and it works great.

Inner tube radius is 19mm but the design is 18mm to account for the ~1mm of "squish" from the print. The tube cap is 22mm and will hold it in place nicely.