Do an angled lap joint. Tough to cut but not beyond most peoples talent. Mark each piece 1' back, scribe a line across. Draw a diagonal line from corner to corner on each piece in that 1' section. Use a saw and cut out the triangular section on each piece so when you slip them together they make the joint. Plane to thickness evenly from each side.

Doing it this way at no point is the wood just 1/2 thickness and where its thin on one piece, its thick on the other.