Free 

I remixed this because I didn't feel like a fff or fdm printed object would be up to holding air in as much as a nylon cap would so I cut out the threads so you can glue in a cheap valve cap. It's remixed from someone on Tinkercad ToaKame “Celtic skull valve cap” and artec3d “Celtic skull”. ( Sorry 'bout the rusty rims, but hey, at least the valve caps look good. )
- .4 mm nozzle - .15 mm layer height - 30% infill - 4 walls sides, top, and bottom - glue in cheap nylon valve cap with E6000
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