Add a second long board to the other side of your assembly. It should come flush with the other butt of the board running across the bottom.

Build a header that will slide into the top of the frame as shown in the image. This is made with two 3' - 3" boards sandwiched around scraps of 1/2" sheet material. Screw this all together with pairs of 3" screws at both ends and a pair in the middle.