I can't find a 350mm OD transparent tube, and I can only imagine that a wood jig to bend a pane into the half cylinder I'd need would need a lot of wood. I'll try and compute a solution later on. For now, I could have a black window for a cinema blackout. Alternatively, I could print a mesh or grid to adhere the film onto. 

Anyway, I've reduced the extra amount of helmet under the door area (the part with the steep chamfer) to increase head rotational freedom. I've also removed the eagle-like front section, so it's now just a cylinder all the way.

If I then add some soft materials, I think it's going to look quite comfortable. I'd still like to avoid a nose cutout on the combiner though. It increases manufacturing complexity, and most likely would still be an issue when I'm lying to the side and the helmet gets pushed to the left/right.