Bal - Chatri traps have been around for a very long time.  I don't know when the first one was created, but I do know it was in India, and was made with horse hair nooses decades ago.  Today, we use a variety of materials for nooses, but I have found a monofilament and knot combination that keeps nooses standing up off the trap instead of laying down flat, and works very well for multiple species of raptors.  Doesn't matter if I set the BC for a Kestrel or a Ferruginous hawk, I know this BC can get the job done.  I came up with the design in 2010, wondering if a flat top would encourage a bird to hop up to a higher surface, as they are prone to do anyway, and to continue running around chasing the bait animal.  I was completely shocked at how well this idea worked.  They will run around on the flat top much longer than they do on a trap with a rounded top.  No matter where they are on top of the trap, they are always directly above the bait animal, and only a couple inches from it.  This keeps the bait animal moving and trying to get away, which keeps the bird moving on top of the trap, and causing it to get caught in the nooses.  Most of the time, there are several nooses on the leg / foot when I get to the trapped raptor.  I have not changed this design in over a decade because it works so well.