I can't share the final engineering drawings but I can share these images which were part of the initial concept drawing set. The production design stanchions were each formed from a single piece of 1/8" steel plate. The only critical dimensions are the height (25.75") and the locations of the bolt holes in the top and bottom flanges, which need to match the holes in the Gr8Tops roof and any holes they put in the tub. The stanchions have two holes in each flange; I don't know if Gr8Tops puts one or two holes in the back of the roof and the tub but these need two holes in order to be rigid enough.
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The first set of stanchions I made as a proof-of-concept were made from steel tubing with mounting flanges welded to the top and bottom, that style might be easier for you to make/have made. Both sets:
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The stanchions are intended to be used with the Safari Cab door surrounds; the surrounds are an important part of the support design for running the roof without the side panels. If you intend to run the roof without the door surrounds then the roof should be tied to the roll bar. The original design included roll bar brackets as shown in this drawing:
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Another way the back of the roof can be supported is with brackets like these to the roll bars - these brackets transfer the weight on the roof rack to the roll bars so no weight is borne by the fiberglass but they also function to support the back of the roof when the sides are off.
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The door surrounds are also off in the photo above; the middle of the roof is supported with brackets just in front of the main hoop of the roll bar - you can see a stainless bolt head there that's holding the bracket to the roll bar and it supports the front of the roof rack as well.