As it is rather short, make it bend in the handle too. You can chase a growth ring for security, but I usually make my board bows from ringporous woods (such as oak) with standing growth rings. The growth rings will then look like straight lines on the back of the bow, with the rings feathering out on the sides of the bow. I'd start with about 30mm width at the middle, tapering down to 12mm at the tips in convex lines.