I've had a small 3" fixed blade knife on a pack strap for years. It does have a sheath that was designed for upside down carry. I've found it to be a very convenient way to carry this blade. I always have at least a SAK in my pocket, so I'm not totally screwed if I were to somehow lose it.

To lash it down, I used 1/8" bungee type cord, and did kind of a crossing pattern like shoelaces. It's very secure with a bit of give because of the stretchy cord. It's sold by the foot. I got it at REI, but I'm sure it's available elsewhere.