Another quick update. Today I fixed the hardboard sheets that will act as my curved screen to the frame, and then mounted the frame on tripod legs to get the screen to the right height. Pictures below.

Half the screen surface in place, frame on the floor

The completed screen up on its tripod legs
This was difficult work to do on my own, given the scale of the frame. I cut the hardboard in-situ downstairs where it was delivered into the smaller pieces I needed – I couldn’t get a single piece, so it had to be done in two halves, about 1570mm x 970mm each. I overcut the length to give myself room to fiddle with the curvature.
I placed the hardboard up against the middle vertical member of the frame, used a clamp to hold it flat in place, and then put in a number of screws down the length of the baton. I moved to the next baton left / right, pushed the hardboard into a curve, adjusted the curve until it appeared circular, and then did the same again, and so on. This left me with a curved screen, a join in the middle (not ideal, but it’s the only way I could do it with the frame as designed), and some overspill on the left and right which I trimmed off.
Then I needed to add the legs. Ordinarily I would just have laid the frame flat on its face, added the legs, and then stood the frame back up again onto the tripod of legs and bingo. But the room I’m building in is just too small for that. I was able to add the middle leg of the tripod, at the very rear of the frame, and still get the frame back to the vertical, but the legs either side were a different question. Eventually, I hit on the answer of screwing the side legs in with a single loose screw, so they could move, and then moved them out of the way to lift the frame, and then moved them back, kickstand-style, once it was up. Then I simply added the second screw and tightened the first, and there you are… one screen, 800mm from the floor, as requested.
Finally, a single bracket was driven into the wall stud and the bottom of the frame, to keep it anchored in place along the wall. I’ll do another along the top later on.
Next steps – add the hangers for the gantry, and the gantry itself. Then filling and painting, then a coat of white primer, and I can start testing the projectors before I finally add the Screen Goo!