Most important part of the fly down is the wing flapping, not really the cackle itself. I have only heard real turkeys make this sound a couple of times, and usually its when they fly a creek or ditch, not when they come out of the tree. Use your hat to smack againest your leg, or thump your fist againest your chest to simulate wing beats, and always scrape some leaves to immitate a three point landing.

I find that the actual cackle part is easiest to imitate on a diaphram call.