Keep an eye on Aldi we got one for about £15 they seem to have them in at least once each year, they also did a decent Box Hitchlock unit for £10.
To be fair I don't often deploy the Wheel Clamp as they are a bit of a faff to fit.
Our main visual deterrent is a 2m length of ships anchor chain, the links are 19mm in diameter thickness and galvanised, I thread that through the "A" Frame of the Trailer, and use a quality CISA Box Padlock.
The above is ideal for locking the trailer onto the car towbar or a nearby tree, when unattended.
Amazing family weekend with old steam engines, classic car displays, market stalls, and full catering and bar. And camping on site - Save £25 by booking in advance.