I used a pattern from this set for my Rob Roy:

http://cgi.ebay.com/PURSE-BAG-BELT-C...item1c169ed950

It needed a little tweeking, but worked out well. In particular, I used the ones on the bottom left. I used deer tanned kid skin for the bag, and veg tanned leather for a stiffener and reenforcer for the front and back.