I ran into the first major problems today. I thought the rest of the build process would be straightforward. I was wrong.
Today's effort was focused on building the top frame of the observatory on top of the walls. I pulled out the two 12 foot long, treated lumber 2x4s, cut them to length and cut the corner miters. I placed them on the walls and the corner posts and two problems became apparent:
1. The boards were slightly warped--the twist was relatively slight (I checked the boards carefully when I bought them and did not notice it). However, anchoring one end of the board really exaggerated the warp at the other end. I thought screwing it down aggressively might straighten the warp, but it didn't. I tried building in 2x4 braces at 3 foot intervals to see if attaching at a number of points might straighten the warp, but that did not work. I even tried using two 6 foot long 2x4s with a center brace, but the solution was not practical because of issue 2, below.
2. The second problem arose from the fact this is a scale-up from smaller sheds. The center wall brace on this shed is 2 inches wide, which "kicks out" the 4x4 corner supports. The top 2x4 frame piece can rest on the edge of these and also on the top of the wall. However, the wall is "corrugated," and it is very difficult indeed to cover the gap caused by the "outward projecting" corrugations and rest enough of the 2x4 on the corner braces. I thought about cutting notches in the corner supports to have them rest more snugly against the wall, but decided that it would be too difficult and might have other consequences when the 4x4 beams and the roof are added to the weight to be supported.
All in all, it was a frustrating afternoon. I achieved precisely nothing and had nothing to show for all the work done today. I think the only practical step is to make the top frame from 2x6s. These will be wide enough to rest on the walls and cover all leak points, and they will also have appropriate overlap with the corner support pillars. Today is the end of build activity for most of the week. I'll buy more timber on Friday (and be very careful to check for warps) and try again. I'm definitely sadder and wiser this evening.