I've found these plastic pipe cutters can make very short work of a variety of materials, including paddle shafts & boat hulls. If you had to cut into a boat to free someone's pinned legs, it would take about a minute with this thing. They weight a couple of ounces and cost about $5 at Home Depot:
I also carry a SAM Splint. Versatile and lightweight - can be used on wrists, ankles, and even as a neck brace is a pinch (if you can't back board the patient):
Nice bit of SEO, by the way...