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
Self-locking nut

Step 2: Disassemble the Wheel

Just remove the axle bolt and take away the wheel that you don't need.

Step 3: Assembling

Drill a hole in the middle of one of the hump on the cross bracket. The hole should allow the bolt of the frame of the swivel caster to fit in.
Lock the bolt with the self-locking nut.
If you want, you can cut the exceeding bolt thread with a grinder before assembling.
Using the grinder, shape a piece of square 30x30mm iron pole as shown in the picture, then drill two passing holes, one for each axle, and fix the piece to the wheel frame by the horizontal axle.
The iron piece will allow the vertical movement, the trailer bar that is going to be attached will allow the horizontal movement and the bearing of the wheel frame the torsional movement.
A simple steel pin will be used to fix the trailer bar to the hitch.
That's it