If these are the typical thickness boards that most cameras such as Deardorf take, (not sure on a B&J) the best material to use is 1/4 inch plywood. The reason being that you have several courses of wood laminated together. This lamination keeps the board from warping in the same way that a single thickness piece of lumber is prone to do. That is why on a lot of factory boards they put two opposing strips at each end. Usually with plywood it is just a matter of cutting the overall dimensions from the plywood and then with the table saw blade lowered to only the lip thickness cutting the lip on the board on each of the sides. If you can not find a lumber yard do this for you, try any cabinet shop, carpenter etc. Anyone with a table saw can cut those out in very short order. Good luck.

Donald MIller