This is a very cheap Hitch made with a swivel caster, a bracket for antenna poles and a piece of square iron pipe.
To be fixed under the bike saddle.
Great just because it allows the movement on three axes, so there's complete freedom while biking.
Step 1: What you need
A swivel caster with bolt. Make sure to buy a wheel that can be removed from the frame, just removing the axle bolt.
A cross bracket for antenna poles