Hi Frank,
I saw your question over on Leatherworker.net as well, but I was hoping someone else would speak up, since I don't really have anything useful for you. Someone over there did make one and give a vague description of how, but I couldn't find it.
My suggestion would be to make a paper pattern--tape paper around the butt and draw the lines to cut on, then cut it out. To cover the end of the butt, you would just need to trace around it on paper and add the thickness of the leather you are using so you can stitch it to the wrap, if that makes any sense?
~Noah