I was going to make a pretty extensive First Aid kit for my 'Field' sports, but I found a Johnson & Johnson First aid kit at Wally-World and supplemented it with some more serious hardware and stored it in an Ammo can.
I like to add things like pre-packaged sutures,
Hemostats, scalpel handle and some sterile scalpel blades,
Eye wash bottle, stuff that the little 'Kits' don't usually have.
Let me see, besides the usual assortment of anti-acids and band-aids,
I have Iodine Solution, probably the best serious wound flush you can have,
I have both Hot & Cold packs,
I have Chem Lights, hard to do first aid in the dark!
Surgical gloves and CPR barrier. You just don't know what people have today...
A big bottle of Tylenol and a healthy tube of anti-biotic ointment,
Topical treatment for poison ivy- sun burns, ect.
A bottle of 'Waterless' anti-microbial hand wash,
Citrus based bug repellant to keep the critters away from a wound...
DON'T FORGET SUN SCREEN! (everyone else will!)
Titanium Oxide based sun screen works best with the least toxic chemicals.
I keep Benadryl in a couple of different forms, good for allergic reactions and bug bites, but will work as a mild sedative.
Also lowers blood pressure for people with heart attacks, stroke or altitude sickness, or just plain flipping out over a situation.
A hand full of the larger 'Telfa' pads so they don't stick to the wounds... They are 4"x4" and a couple of 4"x6".
Usually got a couple of clean garbage bags,
One because I don't like leaving anything in the woods, even first aid packaging,
And TWO, because you can't order the weather.
You may have to dress a serious wound in rain, or try and transport a seriously wounded person out where they are going to get the wound muddy, wet, exposed to insects or sun burn...
Anyway, to keep it all dry and safe when driving, I keep it in an old Ammo Can (repainted with the 'Red Cross'),
When I'm Hiking, I just take the smaller plastic box.