We had a first aid need at a recent Klondike Derby. I took our kit into the warming shelter and left it there. Somehow it walked off. So I need to replace it.

Our kit was an old Johnson and Johnson blue metal box, with supplies being refreshed annually. Since we need to start over, I thought I'd ask for a recommendation. We usually have about 25 at a campout.

The kit in our trailer is an orange plastic "ammo can" with a red cross and "first aid" stenciled on it. Around here most sporting goods or farm/fleet type stores have them for $5-$15

The contents are DIY - one of our moms is a pharmacy tech and takes care of restocking it each spring. I would find a list online as a starting point then think about what supplies you need in the context of a) what level of FA skills do you have and b) what kind of outings do you do (and thus what kind of situations might you encounter). No need to have supplies that you dont know how to use, are not qualified to use or would be very unlikely to ever need.

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.

Although the soft sided packs are nice. I would go with the Plastic Ammo box. With the ammo box it is more waterproof. Yes it is not as organized as the soft sided packs, but being waterproof and rodent proof is nice.