All you need is the boo to be about 1 1/2 wide when it's ready to be glued on a bow. I buy my boo and it comes 2 inches wide and about 3/8th thick. I run it across a jointer to flatten the back side of it till it's a little over 1 1/2 wide. Then I lay my bow pattern out on the flat side and cut that out with a bandsaw. Then I thin that down to 1/8th thick at the handle to 1/16th thick at the tips and glue her on the bow wood.
Hold on a second here Red. My bad:) I thought you were talking about the arch of the boo as it was laying on a table and arching end to end.
But to get the concave out of it, follow what I said above, sorry about that, Roy