Those metal boxes can be secured inside a trailer pretty easily which is nice for knowing where they are, but another option you might consider is looking for a soft-sided type first aid kit. I knew these boxes from scouting, but when I joined the National Ski Patrol started carrying a pretty good kit in my patrol belt (essentially a butt bag). I could lay it down and open it up wide like a book allowing access to everything. I still use a cheap butt bag for my personal FA kit on BW trips, and also have a small soft-sided lunch box/cooler I actually use for my bigger kit in my car. Advantages of soft-sided are lighter and easier to pack, but less protection and depending on type used potentially less easy to access all items inside. Also less easy to affix to the inside of the trailer. Just a thought for you to consider.
For contents, there are several good lists in the general area, and there is also a
First Aid private group just like the scouting group.