there is one piece of first aid equipment that you should have . . .
an oropharyngeal airway , or OPA
basically a curved tube that you place in a ( usually ) unconscious patients' mouth and past their tongue to ensure an airway .
it is the one piece of gear that can save a life on its' own , and would be very difficult , if not impossible to macgyver out of anything else you might have around . splints , bandages , tourniquets you can make out of other shit , but the OPA is very specialized . even someone having a severe allergic reaction to a bee sting ( tongue swelling , compromised airway ) could be kept alive with one .
they come in a variety of sizes , but for an E-kit , just get the largest one and have a means to cut it to length .
of course some training on how to use it is strongly recommended , but at least if you have one , you got that covered .
it's the one piece of first aid gear i always have when going afield .
Add to that a pocket mask. All the para medics I know always state you should always carry one with your first aid stuff.