This morning I tried out my idea for securing the chains on the trailer.
I considered a few options, including as Mike suggested making the chains removeable and then locking them in the trailer tongue box. Although that would have worked, it would have meant coming up with another mechanism, and another lock, to lock the tongue box. Also, the way it is setup a lock on the tongue box likely meant using a padlock - easily defeated with a bolt cutter or a death wheel.
So, I came up with a way to secure the chains using the Bolt lock already on the coupler lock.
The idea is to use a pin welded to the trailer tongue to which the slack ends of the chains could be attached, and then secure it with a sleeve attached to the coupler lock which will slide down over the pin when dropped into place - like this:
After checking the layout on the tongue, I decided it would work. First up, after grinding down the fresh paint, I tacked the sleeve to the back of the coupler lock.
After checking the fit on the tongue, I finished welding it in place. I also added a plate to cover the top of the tube, and to give it a bit more of a finished appearance.,