Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:4578514.

My 2-year-old son wanted to tow things around with his tricycle we got him from a local supermarket for less than $20.

The tricycle has a plastic tow-ball setup.

I modelled this quickly in Creo Parametric and printed it on my CTC Replicator Dual printer.

I've included the Creo Parametric part file if you'd like to adjust sizing etc. Also included the STL file.