Why try to recreate the wheel?
The two viable downhaul tools that I've used with great success are the "Right-It-Right" leverage tool and Chinook's winch (configurations matched to 2-pin cups and the Europin designs). Admittedly, quite different concept approaches, but both work very well. Realistically, the mast extension and universal type can truly influence the best design approach, but the right products are already out there.
Really, the money involved in these products is a drop on the bucket, and well worth the price of admission.