The thing with mechanical security is that you need to add layer after layer to gain more security. Hitch lock, wheel clamp, good chain and lock will each individually slow the thief down by a couple of minutes but combined they present a real deterrent. Research suggests that a thief becomes more nervous the longer he takes to ‘break in’ and equally the more noise he makes. It is thought within the industry that at 3 to 4 minute the thief has almost had enough especially if he is making a lot of noise. It is generally accepted that good security products should be able to resist for 5 minutes, however, it is surprising how few do.
Bad hitch locks can take less than 10 seconds
Bad wheel clamps can take less than 15 seconds, locking wheel nuts and steering wheel devices under a minute.
wheel clamps wheel locks look at drop down
If I was you Ollyit, I would think about getting something welded to the trailer while they are making it. Something like a length of 2”steel box that can be dropped to the ground and locked in place or loops that allow a chain to go around a lamp post and be kept off the ground.
On the subject of chain, unless it is a very good quality security motorcycle chain, forget it, most are soft and can be easily cut with a cheap tool from tool mart.