I can help here because I approach my trailer from knowing that if I don't steal it, someone else will. Lol I also worked retail, tourism and a little construction for years and have seen it all from swapping boots to a spree that took a dozen lawn mowers in less than 3 minutes. I'm also in an area where stolen items either get hauled to Mexico or cut up for scrap by the end of the day. When it's gone, it's gone. Yeah, I can't wait to be able to afford to move.
If you are using a standard ball hitch, put a golf ball in the socket before latching and locking. If you just latch and lock, the trailer can be easily stolen with a 1" bolt in a drop hitch. I demonstrated this to a retail manager after he failed at playing detective. It's a really funny story.
If you also convert to the smaller 1 7/8 or 1.5" ball it will add another level of theft protection because so few trailers use those sizes anymore, therefore, a theif is less likely to also have it.
If I leave my trailer off the vehicle and out of sight, I take a half inch cable and run the ends through each side of the top wheel spoke, through the leaf spring, one side makes a loop around the axle, through the other leaf spring and out through the top spokes again. Sounds difficult but it is easy with stiff cable. Can't pull the trailer, the wheels will bind and catch fire. Can't remove the tire and just unloop because the tire won't fit through the spring and one cable passes around the axle. You can bring a whole new set of tires and drag the old ones but now you just passed from an opportunistic thief to a prepared one. And you can't stop a prepared thief with 2 or more hours.
Also, don't park where a flat bed can just whip in and winch.
As for items within the trailer, I built mine fully enclosed with cattle panels and only carry the big, bulky stuff in it. I currently live in a place that sees sticky fingers pass through so anything of value is typically locked up to some degree. So far it has been working well. Only a few small things have wandered off.
But gotta be realistic, all of these features are for discouraging the honest thieves. With the advent of the battery powered saws, drills and grinders, nothing can't be stolen in less than 10 minutes.