Scratch built is indeed difficult , getting the cross members to form properly was my biggest problem and as you can see the center pole is not quite centered all along .
This unit was made from a broken or separated K-line plastic trailer, the sides are plasticized carton upon which I glued wooden uprights then then attached metal cross bars. Sewing the tarp was perhaps done with a bit too thick thread ( especially for close up photos) the tarp was simply a cloth materiel saturated in artistic paint and left to set on the trailer poles.
Our Canadian stake and racks were a bit different from the American soft top, and or the tarp trailer as the side racks were a good 6 feet in height. This allowed the carrier to ship pretty well any thing that one could put in a van . When the units were empty the racks poles and tarps were taken off so another trailer could be set on top and at times a third flatbed would be added , then the set would be piggybacked to where it had originated saving the trucking company some rail fees.
I remember as a young lad , when it was slow in the office , to go out and help built the racks for several of these units as Speedway began making their own flat bed trailers as it was cheaper than buying from Freauhauf or Trailmobile the tarps came ready made from Canning, the undercarriage from other scrapped units, the rest was made in house ... those were the good old days let me assure you