I have twin boys that have mastered the balance bike but haven't learned the pedal part yet.  So I made this so they could focus on just the pedaling in the house.

It's a 3/8 hole going into the axle so you may need to adjust but it should lift the rear tire about 1in on a 12in bike.

Layer Height: 0.2

Wall Loops: 6

No top or bottom shell layers if you want the Honeycomb look

Infill: 25% - Honeycomb