I don't like the tapered posts idea because it wiggle its way down in transit, jam up solid and soon need a hammer to get them out. Those posts along the sides and ends could have small wider sections to locate onto the tops of the straps (a blacksmith's job), but the drawing is simplified so as not to show them? Or could the straps and the sections of posts below them actually be a housing and the bottom part of the posts are reduced in size to locate in them - i.e. the bottom is part of the housing not part of the post as it appears?
The whole upper structure, including that wide plank, could lift off as one, but why would it?
How high are the sides? I'm thinking whether the loads (not livestock!) could be lifted over?
I truth, I have no idea! But it will be an interesting model.
Mike