A gear I quickly made in blender for a DC motor. The hole in the middle is slightly larger than 2mm in diameter to be able to fit perfectly. It's made to fit perfectly with the same gear so you can create a chain.

I included a screenshot of the size settings in blender so you can see approximately how large it is.

I'm no pro when it comes to printing so your settings might need tweaking. (Also if you have tips on printer settings please contact me and help me :P )