Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:5971344.

These are mason jar inserts for rooting bulb type plants prior to planting. There is a model for both narrow mouth and wide mouth mason jars. You can use them to root, onions, garlic, potatoes, sweet potatoes etc. The onions in the picture have been rooting for ~3 weeks. They cost about 15-20 cents to print and if you have a garden you probably have some old mason jars kicking around.