Fig. 2-6 Base Channel Alignment
Considering the simplicity of the structure, it is wise to begin by bolting together the largest channel pieces. Lay two pieces parallel on a flat surface, close enough together that the remaining two 416 mm channel pieces can lie on top. The openings should be facing towards the center of the square that they form. The picture above (Fig 2-6) indicates the large channel pieces in relation to the wheel mountings and plexi-glass. Use two - four 4mm x 8mm bolts to secure each corner together, and use 4mm nuts to lock them into place.
Next, attach the two DC motor wheel mounts as far front as possible using the 4mm x 38mm bolts and 4mm nuts. You will want to make sure that these are very secure. The picture below shows one of the wheels in question, mounted as far forward as possible. It also shows the arm mounting.