If we were talking about a rifle I'd say, aside from concerns about copying a specific original, as thin as you can. The wood serves no purpose than to act as a handle. I would approach a fowler from that point of view but I am not well versed in fowlers. Still those I really like have been thin, thin, thin in the forearm and with the feel of a beautiful woman's wrist behind and before the lock panel. As thin as the wood and barrel size allows. IMHO