If you want to make sure your fiver isn't stolen, and you don't mind taking the time...
this or
this (locking winter wheels) is ideal.
Yes, it is a pain to remove all the wheels... but a thief could come up with the most advanced metal disintegration device or be the best lockpicker... and that trailer isn't moving, unless the thief provides the wheels and bolts them on.
However, that definitely is a bit extreme for most people. Here is what a friend of mine does with his fiver:
1: He has a kingpin lock on it. It isn't there for security as much as to show a thief that the rig is secured, so someone doesn't try backing up and driving off with it.
2: He has a chain through the wheels, with a real (Abus/Abloy) lock.
3: Before he parked his fiver, he took an augur, dug a fencepost hole about 3-4 feet deep, put a high test security chain (the type that is not cut from the hardware store) with bolts through the links (the bolts were placed on the links that were embedded to ensure the chain would not pull out) partway into the hole, and filled that hole with cement. About 2-3 feet of chain was left protruding from the ground. When the fiver was parked, that chain went around the axle, and was padlocked with a high security padlock.
Needless to say, that fiver didn't move unless he was moving it.