The Spirograph model is great. Printed nicely and without issue. The holes fit the tips of pencils and fine tip markers no problem. 

My son and I were doodling in a notebook and noticed the gear sometimes wanted to rise up out of the track when going around (our notebook paper wasn’t flat… this was not caused by the model design). This prompted me to add a lip over the inner portion of the larger gear guide to help prevent the smaller gears from coming out while doing the patterns. 

The size of the original model is fine but my son wanted one a little bit smaller so we scaled the gears down a bit but remembered to keep the hole size (3.21mm) big enough to still accept pencils and markers. Also maintained the height of 5mm. 

Thank you again Printosaurus.