I just picked up a 7x12 tandem enclosed trailer. I did some searching online on different products/thoughts on how to keep it safe and well, there is a lot to choose from.

What do you guys use? Besides the 45 acp...

 

The trailer coupler on the A-frame, they make a lock cover for this, so a ball cannot be put into the trailer.



The other thing is a Denver boot for one of the wheels

Given these things only work if the trailer is around people. If the trailer is hidden where some one can work on it, they could just take the noise of pulling it on a trailer to haul it away.

 

There is more than just stealing the whole trailer to think about.
Most enclosed cargo trailer use 0.030-0.032" aluminum skin. That means you can break into the trailer through that aluminum skin with a pocket knife. So do NOT keep things you want safe in your trailer.

While storing them if you can block them in with another vehicle is the best way to prevent theft. Preventing breaking into the trailer, the best way is to not use them for storage and keep them unlocked; so unscrouplulus (sp??) people can see inside that there is nothing to steal

If you must store valuable items in your enclosed cargo trailer, there are commercial grade trailers that use heavier skin so that tools would be required to gain entry into the trailer. Trailer manufacturer's like Sharp Manufacturing make trailers with heavier skin that is more difficult to gain unlawful entrance.