Cut all the chassis rails and crossmembers as per the drawing below. Drill the crossmembers as shown. The holes in "Detail A" are drain holes for galvanising and can be omitted if the trailer is to be painted.
"Detail B" holes are for both galvanising drainage and cable routing and should be drilled.
Get your local engineering workshop to fold up the light channel as per the details below. At a pinch you could weld the light channel using individual steel sections. Clamp well and stagger your welding to prevent the light channel buckling and bending.