To get the frames in the right places, I cut a few blocks with accurate 90 deg Angles on my table saw. (A sled helps for this)
Then I clamped these in pairs to the frames, now the frames would sit bolt upright at 90 deg to the scaffold. A roofing square allowed my to check that the frames were square across the scaffold.
I marked off the distances on a long batten and ran that from the transom.
A quick check with Pythagoras ensured me that the difference between measuring along the batten and the strict horizontal distance was less than 1/16 of an inch, so I just ignored it.
The frames are now held in place by these blocks which are pocket hole screwed to the scaffold.
I now need to center the frames left and right and make sure that they are level.
Just to see what it looked like, I cut a half dozen soft wood battens and held them up against the frame notches, They line up pretty well and it starts to look like a boat.
Pictures will follow soon.