The recommended observing slit width is 1/3 the Ø of a dome. 3m / 3 = 1m wide. So I cut two battens to space the slit frame to 1m instead of the former 60cm. [2']
To my mind it is aesthetically displeasing. More like a barn door to let a horse and carriage through. Though it doesn't look quite so bad from inside the dome. I took the image at an angle to show the slit framing ribs more clearly.
I think I'll try spacing the slit ribs at 90cm or ~3' apart just to see what that looks like. It doesn't cost anything except a couple of lengths of batten at this stage.
BTW: None of the slits is supposed to be symmetrical with the dome structure. I just laid the frames over to take some pictures.
I clamped the upper ends of the gores together to hold the dome up. It seems quite stable just relying on friction alone.