This is a cap to put on your model rocket launch pad. It's got a stepped hole inside for 1/4", 3/16", and 1/8" launch rods. 1/8" is the common size that comes with starter kits by Estes, for example. I've played with the hole size a little bit and the current size, printed in PLA, is a snug fit after you work it on and off a few times. If your printer prints inside holes without shrinking them as much as mine, you might tweak the values in the OpenScad file to make them tighter.

The holes at the top are for a flag, a streamer, your launch key, or whatever else you might think of.

I printed in PLA (because that's the material I had in yellow) at 30% infill, which gave it a satisfying heft. It's very visible and obvious, compared to the stock Estes launch cap, but a bit less "pocketable."