First, I used magnets to fit the pieces together to see how they would look in different positions. I found the best place to glue them, and marked it with a pencil. Then I dremeled away the return edges on the back part, so that the parts would fit better together. Return edge over return edge is asking for trouble!
I sanded the pieces down and glued them with ABS weld. Earth magnets are good to use here, and a clamp on each side as well.