Going to build a frame to hold an 8' wide x 4' tall double sided aluminum advertising sign. Going to use 6'' x 6'' treated post 14' tall(4'below ground). Plan to groove 2 x4 or 2 x 6 with a .25'' wide x .50 deep dado to make frame. Going to screw frame to post. How do I join the corners of frame(that joint will be the major stress point)?? Or should I not build frame and groove post to receive sign and use 2 x 6 above and below sign? Any suggestions please.