Start with the fact that you are going to leave a quad and trailer in the middle of nowhere, but where it can be seen, unattended. Then try and minimise that risk. Can you make it less noticeable from the road? Do you really need to leave it there? What alternatives are there? Are you doing it too regularly? Change your routine. A bit of inconvenience can give you more peace of mind. When you've done anything you can in that department, then look at the actual physical security. A simple hitch lock is the first thing. Then secure the bike to the trailer. Then think about alarms. A simple movement alarm which puts out a load of noise will deter many. You could perhaps think about an alarm that calls your mobile. Depending, of course where you are and mobile signals.
But think laterally. Maybe get one of those mines that sets off a blank shotgun cartridge (Henry Krank sells them I think) and use that is one thing that springs to mind. but you get the idea. Be creative about making theft as difficult as possible, with a real danger the thief may get caught and, really that's all you can do. Do you leave it hitched up to your vehicle? A camera looking at the trailer/bike might be good. too.
