I looked at photos from where I painted and looked at the painting itself to figure out where I would put the octagonal shape and what angle. This 3D shape helps with this process SO MUCH.
After I picked the shape I mapped it out using the 3D shape for reference, having to step back from the shape very often to check angles.

Next I added in the lines coming down from the shape using a technique from the three point perspective work I had been doing; using a long ruler to pin point where all the lines met above the painting, and lining this point up with the corners of the shape and mapping out the angle of the lines that way. It was a trial and error process and took lots of removing and re-adding of lines, but I got there in the end!

I was already so much happier with this shape than the last painting! This octangon shape really creates a feeling of being under a great space, even better than the cuboid shape. I considered leaving the shape as it was, but I wanted to add more details out of curiosity, even if it made the painting worse at least I had tried it!
I did some doodle on my phone with an image of the painting to see whether I should add a top to the shape.

After doing this quick digital sketch (above) and adding arches to the shape I decided to add very subtle lines that would converge at the highest point of the shape. But I decided not to show the highest point because I wanted the brushstrokes to look like they were in the structure, so I let the lines go behind the brushstrokes. If I had drawn in all of the roof it would have ruined the illusion of height.

What works
The structure! I think this is the most successful structure shape I have ever done on a painting, and I want to continue pushing this forward! You really feel like you are standing under a structure with great height.
The process that creates the plein air part of the painting. It has taken me this term to really find a process, medium, set of intetions, tools etc that produce paintings that excite me!
This painting shows the potential for next term for painting with this octagonal shape, and for more big plein air paintings to come!
What could be better
Shapes inspired by cathedral interiors. The arches I added remind me too much of a pagoda and not enough of the interior or a cathedral! I need to work on using lines and maybe dots to create more of a sense of the cathedral feeling. Or maybe I pair it back to just the octagonal shape? That is for me to explore next term.