I'll second the Ziploc bag method.
The big kit is totally secondary to the bag of stuff to stop the leaks and plugs the holes - and the majority of that is 4x4 and 6x6 sponges, tape, Sticky Gauze, Shears, Mask and Nitrile Gloves. Under that is a blanket and a towell. I do the current version of CPR - compressions only - and don't need/have the CPR mask anymore.

I used this kit at a motorcycle wreck I happened upon in New York City not too long ago, it did exactly what I needed and best of all the paramedics that showed up replenished my kit for me from their stock right there on the scene!