Weld the angle together to make the frame. The short pieces are sandwiched between the long pieces. See the diagram.

Weld all of the rebar inside of the frame.

It would help to find some 1.5" spacers lying around the shop so you can place them between the rebars quickly and be sure they are square. Nuts and hydraulic fittings might be the right size.

Weld the cross support (the 1/4 x 2 flatbar) in centered and perpendicular to the plane of the frame.