A piece of formica laminate, when broken, produces an amazingly sharp edge that is moderately durable. I have sliced my hand open just like a razor on a small bit of laminate. A small sample chip of laminate is certainly light and unobtrusive. I'd suggest carrying an intact sample chip for best results.

Also, a small ceramic tile sample - while not as tough or light as a laminate sample - will, when broken, produce a failrly tough sharp blade edge, not always as sharp as the laminate, but more durable.

Finally, there's that old stand-by glass. Almost anything made of glass can be broken in a way to produce a very, very sharp edge.