Ditto on the loop that peter mentioned. Both my Radbot and my Superflash attach there easily. Fortunately I haven’t lost a light yet, but I can’t say I haven’t been warned now. It’s usually daylight when I’m riding with my kid in my trailer, so I haven’t worried too much about lights.
Depending on your trailer type and how much of the metal frame is exposed, you may be able either use seat post type light mounts or engineer a way using clamps or zip ties to secure another couple light mounting points on the trailer itself.