In a word: Don’t.
The basic plan was to pile and stake straw bales in the shape of a catenary. Theoretically this provides a nice strong self-supporting vault for the roof and two walls of the building, and then the other two walls can be flat stacked bales. In reality, once the vault is built and not braced by most of the bales inside, it moves a lot. I mean, it stands, and didn’t shift too much in the wind last night, but I opted to set up the tent outside.
The basic problem is forseeable, the junctions between the bales are flexible, and any forces that are not directly down will shift it out of shape. The result is a rather lumpy looking vault - no smooth curve anywhere along it’s length. And that is with a fairly large number of props holding it up on the inside.
Perhaps this could be fixed with better bales (tighter, denser, longer straws), or careful attention to bale placement and plastering as you go. Those aren’t the bales I have though, and cobbing would have added a lot of time (it took me about 16 hours of work), material acquisition, and, most importantly, hindered material reuse if it didn’t work out.
Now I just need to figure out how to get it down.